Migrate backend test suite from xUnit v2 to v3

- Directory.Packages.props: xunit 2.9.3 -> xunit.v3 4.0.0, bump
  xunit.runner.visualstudio to the matching 4.0.0
- All 5 test csproj: xunit -> xunit.v3 package reference, add
  <OutputType>Exe</OutputType> (v3 test projects are standalone
  executables)
- backend/global.json: opt dotnet test into the new MTP-native mode
  (.NET 10 SDK removed the old VSTest bridge for MTP-based test
  projects like xunit v3)
- IAsyncLifetime implementations (HandlerTestBase, ApiTestFixture,
  InfrastructureTestFixture, StorageServiceTests): v3's IAsyncLifetime
  now IS IAsyncDisposable, so InitializeAsync/DisposeAsync return
  ValueTask instead of Task. ApiTestFixture specifically now overrides
  WebApplicationFactory.DisposeAsync instead of implementing
  IAsyncLifetime.DisposeAsync explicitly, since v3 collapses both to
  the same single DisposeAsync slot
- Replaced CancellationToken.None with TestContext.Current.CancellationToken
  across the suite (a v3-only API) and added it to every call that
  was silently omitting a CancellationToken argument, per the new
  xUnit1051 analyzer warnings that came with the v3 package

Verified: dotnet build (0 warnings, 0 errors) and dotnet test, 250/250
passing under the new MTP runner.

backend/Tests/Musify.Infrastructure.Tests/Services/PlayListPresetSeederTests.cs

@@ -45,7 +45,7 @@ public sealed class PlayListPresetSeederTests(InfrastructureTestFixture fixture)
 
     private static async Task RunToCompletionAsync(PlayListPresetSeeder seeder)
     {
-        await seeder.StartAsync(CancellationToken.None);
+        await seeder.StartAsync(TestContext.Current.CancellationToken);
         await (seeder.ExecuteTask ?? Task.CompletedTask);
     }
 
@@ -66,7 +66,7 @@ public sealed class PlayListPresetSeederTests(InfrastructureTestFixture fixture)
                      routes.PresetLargePicturePath
                  })
         {
-            var metadata = await storage.HeadObjectAsync(InfrastructureTestFixture.S3Bucket, key, CancellationToken.None);
+            var metadata = await storage.HeadObjectAsync(InfrastructureTestFixture.S3Bucket, key, TestContext.Current.CancellationToken);
             Assert.True(metadata is not null, $"expected {key} to have been seeded");
         }
     }
@@ -82,7 +82,7 @@ public sealed class PlayListPresetSeederTests(InfrastructureTestFixture fixture)
         await RunToCompletionAsync(secondSeeder);
 
         var metadata = await storage.HeadObjectAsync(
-            InfrastructureTestFixture.S3Bucket, configuration.Routes.PresetSmallPicturePath, CancellationToken.None);
+            InfrastructureTestFixture.S3Bucket, configuration.Routes.PresetSmallPicturePath, TestContext.Current.CancellationToken);
         Assert.NotNull(metadata);
     }
 
@@ -96,7 +96,7 @@ public sealed class PlayListPresetSeederTests(InfrastructureTestFixture fixture)
         await RunToCompletionAsync(seeder);
 
         var metadata = await storage.HeadObjectAsync(
-            InfrastructureTestFixture.S3Bucket, configuration.Routes.PresetSmallPicturePath, CancellationToken.None);
+            InfrastructureTestFixture.S3Bucket, configuration.Routes.PresetSmallPicturePath, TestContext.Current.CancellationToken);
         Assert.Null(metadata);
     }
 }