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/Services/PictureServiceTests.cs

@@ -24,9 +24,9 @@ public sealed class PictureServiceTests
     {
         using var source = CreateSourceImage(800, 600);
 
-        await using var resized = await CreateService().ResizePictureAsWebpAsync(source, 200, 200, CancellationToken.None);
+        await using var resized = await CreateService().ResizePictureAsWebpAsync(source, 200, 200, TestContext.Current.CancellationToken);
 
-        using var result = await Image.LoadAsync(resized);
+        using var result = await Image.LoadAsync(resized, TestContext.Current.CancellationToken);
         Assert.InRange(result.Width, 1, 200);
         Assert.InRange(result.Height, 1, 200);
         Assert.True(result.Width == 200 || result.Height == 200, "expected the longer side to land exactly on the target box");
@@ -37,9 +37,9 @@ public sealed class PictureServiceTests
     {
         using var source = CreateSourceImage(100, 100);
 
-        await using var resized = await CreateService().ResizePictureAsWebpAsync(source, 50, 50, CancellationToken.None);
+        await using var resized = await CreateService().ResizePictureAsWebpAsync(source, 50, 50, TestContext.Current.CancellationToken);
 
-        var format = await Image.DetectFormatAsync(resized);
+        var format = await Image.DetectFormatAsync(resized, TestContext.Current.CancellationToken);
         Assert.Equal("WEBP", format?.Name, ignoreCase: true);
     }
 
@@ -48,9 +48,9 @@ public sealed class PictureServiceTests
     {
         using var source = CreateSourceImage(400, 200);
 
-        await using var resized = await CreateService().ResizePictureAsWebpAsync(source, 100, 100, CancellationToken.None);
+        await using var resized = await CreateService().ResizePictureAsWebpAsync(source, 100, 100, TestContext.Current.CancellationToken);
 
-        using var result = await Image.LoadAsync(resized);
+        using var result = await Image.LoadAsync(resized, TestContext.Current.CancellationToken);
         Assert.Equal(100, result.Width);
         Assert.Equal(50, result.Height);
     }