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/Services/TrackStreamIssuerTests.cs

@@ -22,7 +22,7 @@ public sealed class TrackStreamIssuerTests : HandlerTestBase
         var track = TestEntities.LocalTrack(owner);
         await SeedAsync(owner, track);
 
-        var response = await CreateIssuer().IssueAsync(track.Id, "folder-42", userId: 7, CancellationToken.None);
+        var response = await CreateIssuer().IssueAsync(track.Id, "folder-42", userId: 7, TestContext.Current.CancellationToken);
 
         Assert.Equal("signed-token", response.Ticket);
         Assert.Equal(120, response.ExpiresInSeconds);
@@ -30,7 +30,7 @@ public sealed class TrackStreamIssuerTests : HandlerTestBase
             "https://stream.musify.test/media/Tracks/ProcessedAudios/folder-42/audio.m4a",
             response.ManifestUrl);
 
-        var history = Assert.Single(await Database.ListeningHistories.ToListAsync());
+        var history = Assert.Single(await Database.ListeningHistories.ToListAsync(TestContext.Current.CancellationToken));
         Assert.Equal(7, history.UserId);
         Assert.Equal(track.Id, history.TrackId);
     }
@@ -43,7 +43,7 @@ public sealed class TrackStreamIssuerTests : HandlerTestBase
         var track = TestEntities.LocalTrack(owner);
         await SeedAsync(owner, track);
 
-        await CreateIssuer().IssueAsync(track.Id, "folder-42", userId: 1, CancellationToken.None);
+        await CreateIssuer().IssueAsync(track.Id, "folder-42", userId: 1, TestContext.Current.CancellationToken);
 
         ticketService.Received(1).IssueTicket(1, "Tracks/ProcessedAudios/folder-42/");
     }