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/TestSupport/InfrastructureTestFixture.cs

@@ -44,7 +44,7 @@ public sealed class InfrastructureTestFixture : IAsyncLifetime
 
     public string S3ServiceUrl => $"http://{seaweedFs.Hostname}:{seaweedFs.GetMappedPublicPort(8333)}";
 
-    public async Task InitializeAsync()
+    public async ValueTask InitializeAsync()
     {
         await Task.WhenAll(postgres.StartAsync(), seaweedFs.StartAsync());
 
@@ -56,7 +56,7 @@ public sealed class InfrastructureTestFixture : IAsyncLifetime
         await s3.PutBucketAsync(S3Bucket);
     }
 
-    public async Task DisposeAsync()
+    public async ValueTask DisposeAsync()
     {
         await postgres.DisposeAsync();
         await seaweedFs.DisposeAsync();