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/Mixes/GetMixesByUserIdQueryHandlerTests.cs

@@ -18,7 +18,7 @@ public sealed class GetMixesByUserIdQueryHandlerTests : HandlerTestBase
         var theirs = TestEntities.Mix(other.Id, "Theirs");
         await SeedAsync(owner, other, second, first, theirs);
 
-        var result = await CreateHandler().Handle(new GetMixesByUserIdQuery(owner.Id), CancellationToken.None);
+        var result = await CreateHandler().Handle(new GetMixesByUserIdQuery(owner.Id), TestContext.Current.CancellationToken);
 
         Assert.False(result.IsError);
         Assert.Equal(["First", "Second"], result.Value.Select(mix => mix.Title));
@@ -30,7 +30,7 @@ public sealed class GetMixesByUserIdQueryHandlerTests : HandlerTestBase
         var owner = TestEntities.User();
         await SeedAsync(owner);
 
-        var result = await CreateHandler().Handle(new GetMixesByUserIdQuery(owner.Id), CancellationToken.None);
+        var result = await CreateHandler().Handle(new GetMixesByUserIdQuery(owner.Id), TestContext.Current.CancellationToken);
 
         Assert.False(result.IsError);
         Assert.Empty(result.Value);