McpStdioStateHandler sigterm after grace test is flaky

The `McpStdioStateHandler` unit test `sigterm after grace` intermittently misses the child process output written by its SIGTERM handler.

Example failure: https://github.com/microsoft/vscode/actions/runs/31439865620/job/93622580518

Expected:
```text
stdin ended
SIGTERM received
```

Actual:
```text
stdin ended
```

The test is being disabled until its process termination and output synchronization can be made deterministic.

(Written by Copilot)