Fix: restore task limiter. (#7844)

### What problem does this PR solve?

Close #7828

### Type of change

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

rag/svr/task_executor.py

         with_community = graphrag_conf.get("community", False)
         await run_graphrag(task, task_language, with_resolution, with_community, chat_model, embedding_model, progress_callback)
         progress_callback(prog=1.0, msg="Knowledge Graph done ({:.2f}s)".format(timer() - start_ts))
+        task_limiter = trio.CapacityLimiter(MAX_CONCURRENT_TASKS)
         return
     else:
         # Standard chunking methods