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/Directory.Packages.props

@@ -59,8 +59,8 @@
     <PackageVersion Include="Microsoft.NET.Test.Sdk" Version="17.12.0" />
     <PackageVersion Include="NSubstitute" Version="5.3.0" />
     <PackageVersion Include="coverlet.collector" Version="6.0.4" />
-    <PackageVersion Include="xunit" Version="2.9.3" />
-    <PackageVersion Include="xunit.runner.visualstudio" Version="3.0.2" />
+    <PackageVersion Include="xunit.v3" Version="4.0.0" />
+    <PackageVersion Include="xunit.runner.visualstudio" Version="4.0.0" />
     <PackageVersion Include="Microsoft.AspNetCore.Mvc.Testing" Version="10.0.8" />
     <PackageVersion Include="Testcontainers" Version="4.15.0" />
     <PackageVersion Include="Testcontainers.PostgreSql" Version="4.15.0" />