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.Api.Tests/TestSupport/ApiTestFixture.cs

@@ -37,7 +37,7 @@ public sealed class ApiTestFixture : WebApplicationFactory<Program>, IAsyncLifet
 
     public IStreamTicketService StreamTicketService { get; } = Substitute.For<IStreamTicketService>();
 
-    public async Task InitializeAsync()
+    public async ValueTask InitializeAsync()
     {
         await postgres.StartAsync();
 
@@ -46,7 +46,11 @@ public sealed class ApiTestFixture : WebApplicationFactory<Program>, IAsyncLifet
         await database.Database.MigrateAsync();
     }
 
-    async Task IAsyncLifetime.DisposeAsync()
+    // xUnit v3's IAsyncLifetime IS System.IAsyncDisposable (rather than declaring its own
+    // Task-returning DisposeAsync as in v2), so there is a single DisposeAsync slot to fill —
+    // override the one WebApplicationFactory already provides instead of implementing the
+    // interface explicitly alongside it.
+    public override async ValueTask DisposeAsync()
     {
         await base.DisposeAsync();
         await postgres.DisposeAsync();