Stabilize the batch timeout concurrency test under parallel suite execution

[Widthdom]

Summary

McpServerTests.ProcessFrameAsync_BatchTimedOutActionRetainsConcurrencyLeaseUntilItDrains_Issue4536_Issue4545
can fail with a NullReferenceException at the response-shape assertion when
it runs alongside other tests, while it passes in isolation.

Reproduction evidence

• A full Release solution run failed at McpServerTests.cs:7161.
• A mixed filtered run with query-command tests reproduced the same failure.
• The exact test passed when run alone on the working branch.
• The exact test also passed when run alone from detached origin/main at
  cfd7c776339f7c043d5a0c9287f7140393e5d2b4.

Expected

The signal-gated batch timeout test should remain deterministic under the
repository test suite's parallel execution, or should be isolated from shared
test state that can change the response ordering or shape.