`_reap_stale_sidecar` is `async`, but it identified the processes holding
the sidecar port with two blocking helpers called inline:
* `_find_listener_pids` -> `subprocess.run(["lsof", ...], timeout=5.0)`
* `_pid_is_sidecar` -> `subprocess.run(["ps", ...], timeout=5.0)`, once
per candidate pid
so the inspection can hold the shared gateway loop for 5 + 5·N seconds
while nothing else on it is serviced. It only runs once the /healthz probe
finds something already listening — the orphaned-sidecar recovery path —
and `_reap_stale_sidecar` is awaited from `_start_sidecar`, which runs on
every reconnect (`connect(is_reconnect=True)`). The stall therefore lands
on a live gateway that is still serving every other platform, right when a
crashed sidecar has already left an orphan behind.
Move the whole inspection to one `asyncio.to_thread` hop (one hop rather
than N+1 round trips). The reaping semantics are untouched: SIGTERM for
verified orphans, SIGKILL escalation, and both foreign-listener
RuntimeErrors behave exactly as before.
Same off-the-loop class as the inbound-image decision (#66688) and the
cron-fire verifier.
Adds a regression test asserting both the lsof lookup and the per-pid ps
check execute on a worker thread rather than the loop thread.
a90a2b3c3c