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/Persistence/AuditableEntityInterceptorTests.cs

@@ -14,8 +14,8 @@ public sealed class AuditableEntityInterceptorTests(InfrastructureTestFixture fi
         var before = DateTime.UtcNow;
 
         var user = new User { Id = Random.Shared.NextInt64(1, long.MaxValue), Name = "audit-user", NormalizedName = "AUDIT-USER" };
-        await database.Users.AddAsync(user);
-        await database.SaveChangesAsync(CancellationToken.None);
+        await database.Users.AddAsync(user, TestContext.Current.CancellationToken);
+        await database.SaveChangesAsync(TestContext.Current.CancellationToken);
 
         var after = DateTime.UtcNow;
 
@@ -29,14 +29,14 @@ public sealed class AuditableEntityInterceptorTests(InfrastructureTestFixture fi
         await using var database = fixture.CreateDatabase();
 
         var user = new User { Id = Random.Shared.NextInt64(1, long.MaxValue), Name = "audit-user-2", NormalizedName = "AUDIT-USER-2" };
-        await database.Users.AddAsync(user);
-        await database.SaveChangesAsync(CancellationToken.None);
+        await database.Users.AddAsync(user, TestContext.Current.CancellationToken);
+        await database.SaveChangesAsync(TestContext.Current.CancellationToken);
         var originalCreatedAt = user.CreatedAt;
 
-        await Task.Delay(TimeSpan.FromMilliseconds(50));
+        await Task.Delay(TimeSpan.FromMilliseconds(50), TestContext.Current.CancellationToken);
 
         user.Name = "audit-user-2-renamed";
-        await database.SaveChangesAsync(CancellationToken.None);
+        await database.SaveChangesAsync(TestContext.Current.CancellationToken);
 
         Assert.Equal(originalCreatedAt, user.CreatedAt);
         Assert.True(user.UpdatedAt > originalCreatedAt,