Seal all private classes that are not extended

.NET 7 includes a new analyzer [1] that checks for internal types that
can be sealed. We therefore need to seal all private classes that don't
have derived types to avoid CA1852 compiler warnings on upgrading to
.NET 7.

There's probably a case for also sealing all public types that we don't
intend to extend, but that can be deferred to a separate branch.

[1] dotnet/roslyn-analyzers#5594

src/Buttercup.Web.Tests/Api/MutationTests.cs

@@ -24,7 +24,7 @@ public async Task AuthenticateReturnsPayloadWithSuccessFlagAccessTokenAndUserOnS
     public async Task AuthenticateReturnsPayloadWithNoAccessTokenOrUserOnFailure() =>
         Assert.Equal(new(false, null, null), await new AuthenticateFixture().Authenticate(false));
 
-    private class AuthenticateFixture
+    private sealed class AuthenticateFixture
     {
         public string AccessToken { get; } = "access-token";
 

src/Buttercup.Web.Tests/Authentication/AuthenticationMailerTests.cs

@@ -50,7 +50,7 @@ await fixture.AuthenticationMailer.SendPasswordResetLink(
 
     #endregion
 
-    private class AuthenticationMailerFixture
+    private sealed class AuthenticationMailerFixture
     {
         public AuthenticationMailerFixture() =>
             this.AuthenticationMailer = new(this.MockEmailSender.Object, this.MockLocalizer.Object);

src/Buttercup.Web.Tests/Authentication/AuthenticationManagerTests.cs

@@ -121,7 +121,7 @@ public async Task AuthenticateReturnsNullIfPasswordIsIncorrect()
         Assert.Null(await fixture.Authenticate());
     }
 
-    private class AuthenticateFixture : AuthenticationManagerFixture
+    private sealed class AuthenticateFixture : AuthenticationManagerFixture
     {
         private const string Password = "user-password";
         private const string HashedPassword = "hashed-password";
@@ -299,7 +299,7 @@ public async Task ChangePasswordReturnsTrueOnSuccess()
         Assert.True(await fixture.ChangePassword());
     }
 
-    private class ChangePasswordFixture : AuthenticationManagerFixture
+    private sealed class ChangePasswordFixture : AuthenticationManagerFixture
     {
         private const string CurrentPassword = "current-password";
         private const string HashedCurrentPassword = "hashed-current-password";
@@ -422,7 +422,7 @@ public async Task PasswordResetTokenIsValidReturnsFalseIfInvalid()
         Assert.False(await fixture.PasswordResetTokenIsValid());
     }
 
-    private class PasswordResetTokenFixture : AuthenticationManagerFixture
+    private sealed class PasswordResetTokenFixture : AuthenticationManagerFixture
     {
         private const string Token = "password-reset-token";
 
@@ -563,7 +563,7 @@ public async Task ResetPasswordReturnsUserWithNewSecurityStampOnSuccess()
         Assert.Equal(fixture.User with { SecurityStamp = fixture.NewSecurityStamp }, actual);
     }
 
-    private class ResetPasswordFixture : AuthenticationManagerFixture
+    private sealed class ResetPasswordFixture : AuthenticationManagerFixture
     {
         private const string NewPassword = "new-password";
         private const string Token = "password-reset-token";
@@ -662,7 +662,7 @@ public async Task SendPasswordResetLinkLogsIfEmailIsUnrecognized()
             "password_reset_failure:unrecognized_email", null, fixture.SuppliedEmail);
     }
 
-    private class SendPasswordResetLinkFixture : AuthenticationManagerFixture
+    private sealed class SendPasswordResetLinkFixture : AuthenticationManagerFixture
     {
         private SendPasswordResetLinkFixture(User? user)
         {
@@ -751,7 +751,7 @@ public async Task SignInLogsEvent()
         fixture.AssertAuthenticationEventLogged("sign_in", fixture.User.Id);
     }
 
-    private class SignInFixture : AuthenticationManagerFixture
+    private sealed class SignInFixture : AuthenticationManagerFixture
     {
         public SignInFixture() =>
             this.MockUserPrincipalFactory
@@ -806,7 +806,7 @@ public async Task SignOutDoesNotLogsIfNoUserPreviouslySignedIn()
             x => x.LogEvent(fixture.MySqlConnection, "sign_out", null, null), Times.Never);
     }
 
-    private class SignOutFixture : AuthenticationManagerFixture
+    private sealed class SignOutFixture : AuthenticationManagerFixture
     {
         private SignOutFixture(long? userId) => this.UserId = userId;
 
@@ -919,7 +919,7 @@ public async Task ValidatePrincipalSignsUserOutWhenStampIsIncorrect()
                 null));
     }
 
-    private class ValidatePrincipalFixture : AuthenticationManagerFixture
+    private sealed class ValidatePrincipalFixture : AuthenticationManagerFixture
     {
         private const long UserId = 34;
         private const string UserSecurityStamp = "user-security-stamp";

src/Buttercup.Web.Tests/Authentication/RandomTokenGeneratorTests.cs

@@ -45,7 +45,7 @@ public void GenerateReturnsUrlSafeBase64()
 
     #endregion
 
-    private class RandomTokenGeneratorFixture
+    private sealed class RandomTokenGeneratorFixture
     {
         public RandomTokenGeneratorFixture()
         {

src/Buttercup.Web.Tests/Authentication/TokenAuthenticationServiceTests.cs

@@ -45,7 +45,7 @@ public async Task IssueAccessTokenReturnsToken()
         Assert.Equal(fixture.AccessToken, await fixture.IssueAccessToken());
     }
 
-    private class IssueAccessTokenFixture : TokenAuthenticationServiceFixture
+    private sealed class IssueAccessTokenFixture : TokenAuthenticationServiceFixture
     {
         public IssueAccessTokenFixture() =>
             this.MockAccessTokenEncoder
@@ -149,7 +149,7 @@ public async Task ValidateAccessTokenLogsAndReturnsUserOnSuccess()
             LogLevel.Information, $"Access token successfully validated for user {fixture.User.Id}");
     }
 
-    private class ValidateAccessTokenFixture : TokenAuthenticationServiceFixture
+    private sealed class ValidateAccessTokenFixture : TokenAuthenticationServiceFixture
     {
         private const string AccessToken = "sample-access-token";
 

src/Buttercup.Web.Tests/Controllers/AccountControllerTests.cs

@@ -105,7 +105,7 @@ public async Task ChangePasswordPostRedirectsToYourAccountOnSuccess()
         Assert.Equal(nameof(AccountController.Show), redirectResult.ActionName);
     }
 
-    private class ChangePasswordPostFixture : AccountControllerFixture
+    private sealed class ChangePasswordPostFixture : AccountControllerFixture
     {
         public ChangePasswordPostFixture() =>
             this.MockLocalizer.SetupLocalizedString(

src/Buttercup.Web.Tests/Controllers/AuthenticationControllerTests.cs

@@ -269,7 +269,7 @@ public async Task SignInPostRedirectsDoesNotRedirectToExternalUrl()
         Assert.Equal(nameof(HomeController.Index), redirectResult.ActionName);
     }
 
-    private class SignInPostFixture : AuthenticationControllerFixture
+    private sealed class SignInPostFixture : AuthenticationControllerFixture
     {
         public SignInPostFixture() =>
             this.MockLocalizer.SetupLocalizedString(

src/Buttercup.Web.Tests/Controllers/HomeControllerTests.cs

@@ -39,7 +39,7 @@ public async Task IndexReturnsViewResultWithRecentlyAddedRecipes()
 
     #endregion
 
-    private class HomeControllerFixture : IDisposable
+    private sealed class HomeControllerFixture : IDisposable
     {
         public HomeControllerFixture()
         {

src/Buttercup.Web.Tests/Controllers/RecipesControllerTests.cs

@@ -244,7 +244,7 @@ public async Task DeletePostDeletesRecipeAndRedirectsToIndexPage()
 
     #endregion
 
-    private class RecipesControllerFixture : IDisposable
+    private sealed class RecipesControllerFixture : IDisposable
     {
         public RecipesControllerFixture()
         {

src/Buttercup.Web.Tests/Infrastructure/AssetManifestSourceTests.cs

@@ -47,7 +47,7 @@ public void ProductionManifestCachesResult()
 
     #endregion
 
-    private class AssetManifestSourceFixture
+    private sealed class AssetManifestSourceFixture
     {
         public AssetManifestSourceFixture()
         {

src/Buttercup.Web.Tests/Localization/TimeZoneOptionsHelperTests.cs

@@ -129,7 +129,7 @@ public TimeZoneOptionsHelperFixture() =>
         public TimeZoneOptionsHelper TimeZoneOptionsHelper { get; }
     }
 
-    private class AllOptionsFixture : TimeZoneOptionsHelperFixture
+    private sealed class AllOptionsFixture : TimeZoneOptionsHelperFixture
     {
         private readonly List<TimeZoneInfo> timeZones = new();
 
@@ -153,7 +153,7 @@ public AllOptionsFixture AddFakeTimeZone(
         }
     }
 
-    private class OptionForTimeZoneFixture : TimeZoneOptionsHelperFixture
+    private sealed class OptionForTimeZoneFixture : TimeZoneOptionsHelperFixture
     {
         public string TimeZoneId { get; } = "Sample/Time_Zone";
 

src/Buttercup.Web.Tests/TagHelpers/UserDateTimeTagHelperTests.cs

@@ -71,7 +71,7 @@ public void SetsTitleAttributeToUtcDateTime()
         Assert.Equal("2001-02-03 21:22:23Z", (string)fixture.Output.Attributes["Title"].Value);
     }
 
-    private class UserDateTimeTagHelperFixture
+    private sealed class UserDateTimeTagHelperFixture
     {
         public UserDateTimeTagHelperFixture()
         {