Fix concurrent issue during bundle file concurrent generation testing (…

…halo-dev#6043)

#### What type of PR is this?

/kind failing-test
/area core
/milestone 2.17.x

#### What this PR does / why we need it:

I wrongly invoked `Arraylist#add`(probes) method in multi threads. So the unit test was unstable and might encounter the problem as follows:

```java
Expected :1
Actual   :0
<Click to see difference>

org.opentest4j.AssertionFailedError: expected: <1> but was: <0>
	at org.junit.jupiter.api.AssertionFailureBuilder.build(AssertionFailureBuilder.java:151)
	at org.junit.jupiter.api.AssertionFailureBuilder.buildAndThrow(AssertionFailureBuilder.java:132)
	at org.junit.jupiter.api.AssertEquals.failNotEqual(AssertEquals.java:197)
	at org.junit.jupiter.api.AssertEquals.assertEquals(AssertEquals.java:166)
	at org.junit.jupiter.api.AssertEquals.assertEquals(AssertEquals.java:161)
	at org.junit.jupiter.api.Assertions.assertEquals(Assertions.java:632)
	at run.halo.app.core.extension.service.impl.PluginServiceImplTest$BundleCacheTest.concurrentComputeBundleFileIfAbsent(PluginServiceImplTest.java:460)
```

See https://github.com/halo-dev/halo/actions/runs/9382059472/job/25832681545 for more.

This PR moves the invocation outside thread tasks.

#### Does this PR introduce a user-facing change?

```release-note
None
```

application/src/test/java/run/halo/app/core/extension/service/impl/PluginServiceImplTest.java

@@ -432,17 +432,20 @@ void concurrentComputeBundleFileIfAbsent() {
 
             var probes = new ArrayList<PublisherProbe<DataBuffer>>();
             List<? extends Future<?>> futures = IntStream.range(0, 10)
-                .mapToObj(i -> executorService.submit(() -> {
+                .mapToObj(i -> {
                     var fakeContent = Mono.<DataBuffer>just(sharedInstance.wrap(
                         ("fake-content-" + i).getBytes(UTF_8)
                     ));
                     var probe = PublisherProbe.of(fakeContent);
                     probes.add(probe);
-                    cache.computeIfAbsent("fake-version", probe.mono())
-                        .as(StepVerifier::create)
-                        .expectNextCount(1)
-                        .verifyComplete();
-                }))
+                    return executorService.submit(
+                        () -> {
+                            cache.computeIfAbsent("fake-version", probe.mono())
+                                .as(StepVerifier::create)
+                                .expectNextCount(1)
+                                .verifyComplete();
+                        });
+                })
                 .toList();
             executorService.shutdown();
             futures.forEach(future -> {