[WIP] Fix flaky task cancellation test hang on Windows Debug CI (#1701)

[PranavSenthilnathan]

Summary

Fixes #1701 — TaskTool_CancellationToken_GetTaskShowsWorkingBeforeCancel intermittently causes the xUnit test host to hang for 7+ minutes on Windows Debug CI.

Root Cause

Two issues combine to cause the hang:

1. Test doesn't wait for background tool cancellation before teardown

TaskTool_CancellationToken_GetTaskShowsWorkingBeforeCancel ends with:

await client.CancelTaskAsync(taskId, ct);
// <-- returns here; background Task.Run may still be live

CancelTaskAsync signals the CTS but doesn't wait for the tool body to finish. The sibling test (TaskTool_CancellationToken_FiresWhenExplicitlyCancelled) correctly awaits _toolCancellationFired.Task.WaitAsync(...) first — the flaky test was missing this.

2. Background Task.Run not tracked during server disposal

When a TaskStore is configured, McpServerImpl fires off background Task.Run lambdas to run tools but never tracked or awaited them in DisposeAsync. Under the MRTR-aware path (which is active on the 2026-07-28 protocol with a stateful transport), ObserveHandlerCompletionAsync must decrement _mrtrInFlightCount before DisposeAsync can complete. On Windows in Debug builds, thread-pool scheduling pressure can delay that decrement, causing DisposeAsync to wait on _allMrtrHandlersCompleted — and if the background task itself is also delayed, this manifests as a multi-minute hang.

Fixes

Fix 1 (test): Add await _toolCancellationFired.Task.WaitAsync(TestConstants.DefaultTimeout, ct) after CancelTaskAsync in the flaky test, matching the pattern used by the passing sibling test.

Fix 2 (infrastructure): Track each background Task.Run in _backgroundTasks: ConcurrentDictionary<Task, byte> and drain it at the end of DisposeAsync (after the CTSs are cancelled). This makes server disposal deterministic — even if a test skips the wait, the server won't tear down while a background runner is still alive.

Testing

• All 3 TaskCancellation* tests pass on .NET 8, 9, 10 (they are skipped on .NET Framework 4.7.2 by the existing #if !NET guard tracking issue Running ModelContextProtocol.Tests in net472 is causing VS test platform to hang #587)
• Zero new warnings or build errors