fix: Fix the problem that concurrent execution limit in task executor fails and causes OOM (issue#7580) (#7700)

### What problem does this PR solve?

## Cause of the bug:
During the execution process, due to improper use of trio
CapacityLimiter, the configuration parameter MAX_CONCURRENT_TASKS is
invalid, causing the executor to take out a large number of tasks from
the Redis queue at one time.

This behavior will cause the task executor to occupy too much memory and
be killed by the OS when a large number of tasks exist at the same time.
As a result, all executing tasks are suspended.

## Fix:
Added the task_manager method to the entry of /rag/svr/task_executor.py
to make CapacityLimiter effective. Deleted the invalid async with
statement.

## Fix result:
After testing, the task executor execution meets expectations, that is:
concurrent execution of up to $MAX_CONCURRENT_TASKS tasks.

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)
- [ ] New Feature (non-breaking change which adds functionality)
- [ ] Documentation Update
- [ ] Refactoring
- [ ] Performance Improvement
- [ ] Other (please describe):

rag/svr/task_executor.py

@@ -713,7 +713,10 @@ def recover_pending_tasks():
             redis_lock.release()
             stop_event.wait(60)
         
-
+async def task_manager():
+    global task_limiter
+    async with task_limiter:
+        await handle_task()
 
 
 async def main():
@@ -742,8 +745,8 @@ async def main():
     async with trio.open_nursery() as nursery:
         nursery.start_soon(report_status)
         while not stop_event.is_set():
-            async with task_limiter:
-                nursery.start_soon(handle_task)
+            nursery.start_soon(task_manager)
+            await trio.sleep(0.1)
     logging.error("BUG!!! You should not reach here!!!")
 
 if __name__ == "__main__":