Add an explicit _closing guard to both owned executors so the
recreate-on-shutdown path only recovers from an *external* teardown of
the loop default — never resurrects a pool the gateway/adapter itself
stopped. _shutdown_*executor() sets the flag; _get_*executor() raises if
closing; feishu connect() re-arms on reconnect. Updates the gateway
recreate test to assert the refusal contract and adds feishu coverage.
Feishu SDK calls ran on asyncio's shared default executor, so a torn-down
default executor wedged every send with 'Executor shutdown has been called'
and left the gateway a zombie (#10849). The adapter now owns a
ThreadPoolExecutor recreated on demand if shut down, mirroring the
gateway-owned executor change. Routes all 17 self._client SDK calls through
_run_blocking; shuts the pool down on disconnect.