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.
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