Add await to release main worker thread

Author: sjmonson

src/guidellm/scheduler/worker.py

@@ -190,6 +190,7 @@ async def _process_runner():
             pending_sessions: list[RequestSession[RequestT, ResponseT]] = []
 
             while True:  # TODO: Exit condition
+                await asyncio.sleep(0)  # Yield control to the event loop
                 await lock.acquire()
 
                 request_session = None