fix(worker): ensure safe iteration over process pool during job joining (livekit#3710)

Author: Panmax

livekit-agents/livekit/agents/worker.py

@@ -541,7 +541,7 @@ async def drain(self, timeout: int | None = None) -> None:
         await self._update_worker_status()
 
         async def _join_jobs() -> None:
-            for proc in self._proc_pool.processes:
+            for proc in list(self._proc_pool.processes):
                 if proc.running_job:
                     await proc.join()