Fix: add executor.shutdown (#14403)

### What problem does this PR solve?

Add executor shutdown in finally clause to free resources.

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)

rag/svr/task_executor.py

@@ -1289,6 +1289,7 @@ async def do_handle_task(task):
         )
 
     finally:
+        executor.shutdown(wait=False)
         if has_canceled(task_id):
             try:
                 exists = await thread_pool_exec(