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.Application.Tests/YouTube/SearchYouTubeTracksQueryHandlerTests.cs

@@ -15,11 +15,11 @@ public sealed class SearchYouTubeTracksQueryHandlerTests
     [Fact]
     public async Task Handle_EmptyQuery_ReturnsValidationErrorWithoutCallingTheService()
     {
-        var result = await CreateHandler().Handle(new SearchYouTubeTracksQuery(string.Empty, string.Empty), CancellationToken.None);
+        var result = await CreateHandler().Handle(new SearchYouTubeTracksQuery(string.Empty, string.Empty), TestContext.Current.CancellationToken);
 
         Assert.Equal(ErrorType.Validation, result.FirstError.Type);
         await youTubeMusicService.DidNotReceiveWithAnyArgs()
-            .SearchSongsAsync(default!, default!, default);
+            .SearchSongsAsync(default!, default!, TestContext.Current.CancellationToken);
     }
 
     [Fact]
@@ -28,7 +28,7 @@ public sealed class SearchYouTubeTracksQueryHandlerTests
         var expected = new YouTubeSearchResult([], "next-token");
         youTubeMusicService.SearchSongsAsync("query", "token", Arg.Any<CancellationToken>()).Returns(expected);
 
-        var result = await CreateHandler().Handle(new SearchYouTubeTracksQuery("query", "token"), CancellationToken.None);
+        var result = await CreateHandler().Handle(new SearchYouTubeTracksQuery("query", "token"), TestContext.Current.CancellationToken);
 
         Assert.False(result.IsError);
         Assert.Same(expected, result.Value);