test: CGS discard tests (#38206)

## Description
> [!TIP]  
> _Add a TL;DR when the description is longer than 500 words or
extremely technical (helps the content, marketing, and DevRel team)._
>
> _Please also include relevant motivation and context. List any
dependencies that are required for this change. Add links to Notion,
Figma or any other documents that might be relevant to the PR._


Fixes #`Issue Number`  
_or_  
Fixes `Issue URL`
> [!WARNING]  
> _If no issue exists, please create an issue first, and check with the
maintainers if the issue is valid._

## Automation

/ok-to-test tags=""

### 🔍 Cypress test results
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> If you modify the content in this section, you are likely to disrupt
the CI result for your PR.

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## Communication
Should the DevRel and Marketing teams inform users about this change?
- [ ] Yes
- [ ] No


<!-- This is an auto-generated comment: release notes by coderabbit.ai
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## Summary by CodeRabbit

- **Tests**
- Introduced a new test suite for the Git discard functionality,
covering various scenarios to ensure application integrity during Git
operations.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

app/server/appsmith-server/src/test/java/com/appsmith/server/git/ops/GitDiscardTests.java

@@ -0,0 +1,293 @@
+package com.appsmith.server.git.ops;
+
+import com.appsmith.external.dtos.GitStatusDTO;
+import com.appsmith.external.dtos.MergeStatusDTO;
+import com.appsmith.external.git.handler.FSGitHandler;
+import com.appsmith.server.applications.base.ApplicationService;
+import com.appsmith.server.constants.ArtifactType;
+import com.appsmith.server.domains.Application;
+import com.appsmith.server.domains.GitArtifactMetadata;
+import com.appsmith.server.domains.GitAuth;
+import com.appsmith.server.domains.GitProfile;
+import com.appsmith.server.domains.User;
+import com.appsmith.server.domains.Workspace;
+import com.appsmith.server.dtos.ApplicationJson;
+import com.appsmith.server.dtos.ArtifactExchangeJson;
+import com.appsmith.server.dtos.GitConnectDTO;
+import com.appsmith.server.dtos.PageDTO;
+import com.appsmith.server.git.central.CentralGitService;
+import com.appsmith.server.git.central.GitHandlingService;
+import com.appsmith.server.git.central.GitType;
+import com.appsmith.server.helpers.CommonGitFileUtils;
+import com.appsmith.server.helpers.MockPluginExecutor;
+import com.appsmith.server.helpers.PluginExecutorHelper;
+import com.appsmith.server.migrations.JsonSchemaMigration;
+import com.appsmith.server.services.ApplicationPageService;
+import com.appsmith.server.services.UserService;
+import com.appsmith.server.services.WorkspaceService;
+import com.fasterxml.jackson.databind.MapperFeature;
+import com.fasterxml.jackson.databind.ObjectMapper;
+import org.apache.commons.lang.StringUtils;
+import org.eclipse.jgit.api.errors.GitAPIException;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.TestInstance;
+import org.mockito.Mockito;
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.boot.test.context.SpringBootTest;
+import org.springframework.boot.test.mock.mockito.SpyBean;
+import org.springframework.core.io.ClassPathResource;
+import org.springframework.security.test.context.support.WithUserDetails;
+import reactor.core.publisher.Mono;
+import reactor.test.StepVerifier;
+
+import java.io.IOException;
+import java.nio.charset.Charset;
+import java.nio.file.Path;
+import java.nio.file.Paths;
+import java.time.Duration;
+import java.util.UUID;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.mockito.ArgumentMatchers.any;
+import static org.mockito.ArgumentMatchers.eq;
+
+@SpringBootTest
+@TestInstance(TestInstance.Lifecycle.PER_CLASS)
+public class GitDiscardTests {
+
+    @Autowired
+    CentralGitService centralGitService;
+
+    @Autowired
+    ApplicationService applicationService;
+
+    @Autowired
+    ApplicationPageService applicationPageService;
+
+    @Autowired
+    WorkspaceService workspaceService;
+
+    @Autowired
+    UserService userService;
+
+    @Autowired
+    ObjectMapper objectMapper;
+
+    @Autowired
+    JsonSchemaMigration jsonSchemaMigration;
+
+    @SpyBean
+    FSGitHandler fsGitHandler;
+
+    @SpyBean
+    CommonGitFileUtils commonGitFileUtils;
+
+    @SpyBean
+    GitHandlingService gitHandlingService;
+
+    @SpyBean
+    PluginExecutorHelper pluginExecutorHelper;
+
+    private Application createApplicationConnectedToGit(String name, String branchName, String workspaceId)
+            throws IOException {
+
+        Mockito.doReturn(Mono.just(new MockPluginExecutor()))
+                .when(pluginExecutorHelper)
+                .getPluginExecutor(any());
+
+        if (StringUtils.isEmpty(branchName)) {
+            branchName = "foo";
+        }
+        Mockito.doReturn(Mono.just(branchName))
+                .when(fsGitHandler)
+                .cloneRemoteIntoArtifactRepo(any(), Mockito.anyString(), Mockito.anyString(), Mockito.anyString());
+        Mockito.doReturn(Mono.just("commit"))
+                .when(fsGitHandler)
+                .commitArtifact(
+                        any(Path.class),
+                        Mockito.anyString(),
+                        Mockito.anyString(),
+                        Mockito.anyString(),
+                        Mockito.anyBoolean(),
+                        Mockito.anyBoolean());
+        Mockito.doReturn(Mono.just(true)).when(fsGitHandler).checkoutToBranch(any(Path.class), Mockito.anyString());
+        Mockito.doReturn(Mono.just("success"))
+                .when(fsGitHandler)
+                .pushApplication(any(Path.class), any(), any(), any(), any());
+        Mockito.doReturn(Mono.just(true)).when(commonGitFileUtils).checkIfDirectoryIsEmpty(any(Path.class));
+        Mockito.doReturn(Mono.just(Paths.get("textPath")))
+                .when(commonGitFileUtils)
+                .initializeReadme(any(Path.class), Mockito.anyString(), Mockito.anyString());
+        Mockito.doReturn(Mono.just(Paths.get("path")))
+                .when(commonGitFileUtils)
+                .saveArtifactToLocalRepoWithAnalytics(any(Path.class), any(), Mockito.anyString());
+
+        Application testApplication = new Application();
+        testApplication.setName(name);
+        testApplication.setWorkspaceId(workspaceId);
+        Application application1 =
+                applicationPageService.createApplication(testApplication).block();
+
+        GitArtifactMetadata gitArtifactMetadata = new GitArtifactMetadata();
+        GitAuth gitAuth = new GitAuth();
+        gitAuth.setPublicKey("testkey");
+        gitAuth.setPrivateKey("privatekey");
+        gitArtifactMetadata.setGitAuth(gitAuth);
+        gitArtifactMetadata.setDefaultApplicationId(application1.getId());
+        gitArtifactMetadata.setRepoName("testRepo");
+        application1.setGitApplicationMetadata(gitArtifactMetadata);
+        application1 = applicationService.save(application1).block();
+
+        PageDTO page = new PageDTO();
+        page.setName("New Page");
+        page.setApplicationId(application1.getId());
+        applicationPageService.createPage(page).block();
+
+        GitProfile gitProfile = new GitProfile();
+        gitProfile.setAuthorEmail("[email protected]");
+        gitProfile.setAuthorName("testUser");
+        GitConnectDTO gitConnectDTO = new GitConnectDTO();
+        gitConnectDTO.setRemoteUrl("[email protected]:test/testRepo.git");
+        gitConnectDTO.setGitProfile(gitProfile);
+        return centralGitService
+                .connectArtifactToGit(
+                        application1.getId(), gitConnectDTO, "baseUrl", ArtifactType.APPLICATION, GitType.FILE_SYSTEM)
+                .map(artifact -> (Application) artifact)
+                .block();
+    }
+
+    private Mono<? extends ArtifactExchangeJson> createArtifactJson(String filePath) throws IOException {
+
+        ClassPathResource classPathResource = new ClassPathResource(filePath);
+
+        String artifactJson = classPathResource.getContentAsString(Charset.defaultCharset());
+
+        Class<? extends ArtifactExchangeJson> exchangeJsonType = ApplicationJson.class;
+
+        ArtifactExchangeJson artifactExchangeJson =
+                objectMapper.copy().disable(MapperFeature.USE_ANNOTATIONS).readValue(artifactJson, exchangeJsonType);
+
+        return jsonSchemaMigration.migrateArtifactExchangeJsonToLatestSchema(artifactExchangeJson, null, null);
+    }
+
+    @Test
+    @WithUserDetails(value = "api_user")
+    public void discardChanges_whenUpstreamChangesAvailable_discardsSuccessfully() throws IOException {
+        User apiUser = userService.findByEmail("api_user").block();
+        Workspace toCreate = new Workspace();
+        toCreate.setName("Workspace_" + UUID.randomUUID());
+        Workspace workspace =
+                workspaceService.create(toCreate, apiUser, Boolean.FALSE).block();
+        assertThat(workspace).isNotNull();
+
+        Application application =
+                createApplicationConnectedToGit("discard-changes", "discard-change-branch", workspace.getId());
+        MergeStatusDTO mergeStatusDTO = new MergeStatusDTO();
+        mergeStatusDTO.setStatus("2 commits pulled");
+        mergeStatusDTO.setMergeAble(true);
+
+        ArtifactExchangeJson artifactExchangeJson = createArtifactJson(
+                        "test_assets/ImportExportServiceTest/valid-application-without-action-collection.json")
+                .block();
+        ((Application) artifactExchangeJson.getArtifact()).setName("discardChangesAvailable");
+
+        GitStatusDTO gitStatusDTO = new GitStatusDTO();
+        gitStatusDTO.setAheadCount(2);
+        gitStatusDTO.setBehindCount(0);
+        gitStatusDTO.setIsClean(true);
+
+        Mockito.doReturn(Mono.just(Paths.get("path")))
+                .when(commonGitFileUtils)
+                .saveArtifactToLocalRepoWithAnalytics(any(Path.class), any(), Mockito.anyString());
+        Mockito.doReturn(Mono.just(artifactExchangeJson))
+                .when(gitHandlingService)
+                .recreateArtifactJsonFromLastCommit(Mockito.any());
+        Mockito.doReturn(Mono.just(true)).when(fsGitHandler).rebaseBranch(any(Path.class), Mockito.anyString());
+
+        Mono<Application> applicationMono = centralGitService
+                .discardChanges(application.getId(), ArtifactType.APPLICATION, GitType.FILE_SYSTEM)
+                .map(artifact -> (Application) artifact);
+
+        StepVerifier.create(applicationMono)
+                .assertNext(application1 -> {
+                    assertThat(application1.getPages()).isNotEqualTo(application.getPages());
+                })
+                .verifyComplete();
+    }
+
+    @Test
+    @WithUserDetails(value = "api_user")
+    public void discardChanges_whenCancelledMidway_discardsSuccessfully() throws IOException, GitAPIException {
+        User apiUser = userService.findByEmail("api_user").block();
+        Workspace toCreate = new Workspace();
+        toCreate.setName("Workspace_" + UUID.randomUUID());
+        Workspace workspace =
+                workspaceService.create(toCreate, apiUser, Boolean.FALSE).block();
+        assertThat(workspace).isNotNull();
+
+        Application application = createApplicationConnectedToGit(
+                "discard-changes-midway", "discard-change-midway-branch", workspace.getId());
+        MergeStatusDTO mergeStatusDTO = new MergeStatusDTO();
+        mergeStatusDTO.setStatus("Nothing to fetch from remote. All changes are upto date.");
+        mergeStatusDTO.setMergeAble(true);
+
+        ArtifactExchangeJson artifactExchangeJson = createArtifactJson(
+                        "test_assets/ImportExportServiceTest/valid-application-without-action-collection.json")
+                .block();
+        ((Application) artifactExchangeJson.getArtifact()).setName("discard-changes-midway");
+
+        GitStatusDTO gitStatusDTO = new GitStatusDTO();
+        gitStatusDTO.setAheadCount(0);
+        gitStatusDTO.setBehindCount(0);
+        gitStatusDTO.setIsClean(true);
+
+        Mockito.doReturn(Mono.just(Paths.get("path")))
+                .when(commonGitFileUtils)
+                .saveArtifactToLocalRepoWithAnalytics(any(Path.class), any(), Mockito.anyString());
+        Mockito.doReturn(Mono.just(artifactExchangeJson))
+                .when(gitHandlingService)
+                .recreateArtifactJsonFromLastCommit(Mockito.any());
+        Mockito.doReturn(Mono.just(mergeStatusDTO))
+                .when(fsGitHandler)
+                .pullApplication(
+                        any(Path.class),
+                        Mockito.anyString(),
+                        Mockito.anyString(),
+                        Mockito.anyString(),
+                        Mockito.anyString());
+        Mockito.doReturn(Mono.just(gitStatusDTO)).when(fsGitHandler).getStatus(any(Path.class), Mockito.anyString());
+        Mockito.doReturn(Mono.just("fetched"))
+                .when(fsGitHandler)
+                .fetchRemote(
+                        any(Path.class),
+                        Mockito.anyString(),
+                        Mockito.anyString(),
+                        eq(true),
+                        Mockito.anyString(),
+                        Mockito.anyBoolean());
+
+        centralGitService
+                .discardChanges(application.getId(), ArtifactType.APPLICATION, GitType.FILE_SYSTEM)
+                .map(artifact -> (Application) artifact)
+                .timeout(Duration.ofNanos(100))
+                .subscribe();
+
+        // Wait for git clone to complete
+        Mono<Application> applicationFromDbMono = Mono.just(application).flatMap(application1 -> {
+            try {
+                // Before fetching the git connected application, sleep for 5 seconds to ensure that the clone
+                // completes
+                Thread.sleep(5000);
+            } catch (InterruptedException e) {
+                e.printStackTrace();
+            }
+            return applicationService.getById(application1.getId());
+        });
+
+        StepVerifier.create(applicationFromDbMono)
+                .assertNext(application1 -> {
+                    assertThat(application1).isNotEqualTo(application);
+                })
+                .verifyComplete();
+    }
+}