Follow-up on the cherry-picked #13173 fix. Holds the _run_restart task in
self._restart_task (a bare asyncio.create_task keeps only a weak reference,
so a still-pending task can be GC'd mid-flight) and explicitly skips it in
the _stop_impl cancel loop alongside _stop_task. Adds AUTHOR_MAP entry for
the contributor and a regression test that fails when the task is cancellable.
Refs #12875
f2ca3e3d84