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/Users/SyncUserCommandHandlerTests.cs

@@ -13,10 +13,10 @@ public sealed class SyncUserCommandHandlerTests : HandlerTestBase
     {
         var command = new SyncUserCommand(1, "Jane Doe", "Jane", "Doe", "https://avatar.example/jane.png");
 
-        var result = await CreateHandler().Handle(command, CancellationToken.None);
+        var result = await CreateHandler().Handle(command, TestContext.Current.CancellationToken);
 
         Assert.False(result.IsError);
-        var stored = await Database.Users.FindAsync(1L);
+        var stored = await Database.Users.FindAsync([1L], TestContext.Current.CancellationToken);
         Assert.NotNull(stored);
         Assert.Equal("Jane Doe", stored.Name);
         Assert.Equal("https://avatar.example/jane.png", stored.ProfilePictureUrl);
@@ -30,10 +30,10 @@ public sealed class SyncUserCommandHandlerTests : HandlerTestBase
 
         var command = new SyncUserCommand(1, "New Name", "New", "Name", "https://avatar.example/new.png");
 
-        var result = await CreateHandler().Handle(command, CancellationToken.None);
+        var result = await CreateHandler().Handle(command, TestContext.Current.CancellationToken);
 
         Assert.False(result.IsError);
-        var stored = await Database.Users.FindAsync(1L);
+        var stored = await Database.Users.FindAsync([1L], TestContext.Current.CancellationToken);
         Assert.NotNull(stored);
         Assert.Equal("New Name", stored.Name);
         Assert.Equal("NEW NAME", stored.NormalizedName);