## What changed
- Guard executor-backed MCP stdio sends with a single-permit semaphore so a
second JSON-RPC message cannot start writing while the first write is pending.
- Add a regression test that drives two concurrent sends and verifies the
process receives each newline-delimited message in order without overlap.
GitOrigin-RevId: 30c2e51827ecf57b5eb23d9ad1e876734d6e784e