MTP-hosted xUnit v3 in VS Test Explorer: TestContext.Current.CancellationToken and RunTestCases token both fail to fire on Stop

[MhKarapetyan]

Under xUnit v3 hosted by Microsoft Testing Platform in VS 2026 18.5.x's
DevHub-hosted Test Explorer, pressing Stop Test Run does not fire any
cancellation token that the test code or a custom XunitTestFrameworkExecutor
override can observe.

How to reproduce

1. dotnet build this folder (or open StopRepro.csproj in VS).
2. In VS Test Explorer, Run Token_Should_Fire_When_Test_Explorer_Stop_Is_Pressed.
3. After ~5 seconds (you'll see poll heartbeats appearing in the log), press
   Stop Test Run in Test Explorer.
4. Wait at least 30 seconds. Do not stop VS / close anything else.
5. Open the log file:

   %TEMP%\stop_repro.log
   

Observed output (bug)

None of those appears. The log only contains:

… [Executor] RunTestCases token: CanBeCanceled=True, AlreadyCancelled=False
… [Test] TEST STARTED. …
… [Test] TestContext token: CanBeCanceled=True, AlreadyCancelled=False
… [Test] poll #10, IsCancellationRequested=False
… [Test] poll #20, IsCancellationRequested=False
…
… [Test] TIMED OUT after N polls. Token never fired. 

[StopRepro.zip](https://github.com/user-attachments/files/28453115/StopRepro.zip)

[bradwilson]

Waiting for a response from Microsoft on #447 (comment)