Refactor: tidy up ThreadPoolExecutor lifecycle in file_service and task executor (#14668)

## Summary
- Wrap the `ThreadPoolExecutor` instances in `FileService.parse_docs`
and `FileService.get_files` with `with ... as exe:` blocks for
deterministic cleanup
- Replace the `concurrent.futures.ThreadPoolExecutor` in
`do_handle_task` with `asyncio.create_task(asyncio.to_thread(build_TOC,
...))`, preserving the existing parallelism with chunk insertion while
leveraging the surrounding async context
- Drop the now-unused `import concurrent` and the
`executor.shutdown(wait=False)` call in the `finally` block

Closes #14622.

No behavioral change, no public API change. Net diff: ~19 insertions /
25 deletions across two files.

## Test plan
- [ ] `uv run ruff check api/db/services/file_service.py
rag/svr/task_executor.py` passes
- [ ] Upload a multi-file batch through the chat/file endpoint and
confirm `FileService.parse_docs` still returns combined parsed text
- [ ] Trigger `FileService.get_files` via the chat reference flow with a
mix of image and non-image files; verify both `raw=True` and `raw=False`
paths return correctly
- [ ] Run a `naive`-parser document task with `toc_extraction: true` and
confirm the TOC chunk is generated and inserted exactly as before
- [ ] Run a `naive`-parser document task with `toc_extraction: false`
and confirm the path with `toc_thread = None` is unaffected
- [ ] Cancel a running task to exercise the `finally` block and confirm
cleanup still works without the executor shutdown call

---------

Co-authored-by: web-dev0521 <jasonpette1783@gmail.com>
Co-authored-by: Wang Qi <wangq8@outlook.com>

rag/svr/task_executor.py

@@ -22,7 +22,6 @@ start_ts = time.time()
 
 import asyncio
 import socket
-import concurrent
 # from beartype import BeartypeConf
 # from beartype.claw import beartype_all  # <-- you didn't sign up for this
 # beartype_all(conf=BeartypeConf(violation_type=UserWarning))    # <-- emit warnings from all code
@@ -1089,7 +1088,6 @@ async def do_handle_task(task):
     task_parser_config = task["parser_config"]
     task_start_ts = timer()
     toc_thread = None
-    executor = concurrent.futures.ThreadPoolExecutor()
 
     # prepare the progress callback function
     progress_callback = partial(set_progress, task_id, task_from_page, task_to_page)
@@ -1251,7 +1249,7 @@ async def do_handle_task(task):
         logging.info(progress_message)
         progress_callback(msg=progress_message)
         if task["parser_id"].lower() == "naive" and task["parser_config"].get("toc_extraction", False):
-            toc_thread = executor.submit(build_TOC, task, chunks, progress_callback)
+            toc_thread = asyncio.create_task(asyncio.to_thread(build_TOC, task, chunks, progress_callback))
 
     chunk_count = len(set([chunk["id"] for chunk in chunks]))
     start_ts = timer()
@@ -1318,7 +1316,7 @@ async def do_handle_task(task):
         progress_callback(msg="Indexing done ({:.2f}s).".format(timer() - start_ts))
 
         if toc_thread:
-            d = toc_thread.result()
+            d = await toc_thread
             if d:
                 if not await _maybe_insert_chunks([d]):
                     return
@@ -1337,7 +1335,8 @@ async def do_handle_task(task):
         )
 
     finally:
-        executor.shutdown(wait=False)
+        if toc_thread is not None and not toc_thread.done():
+            toc_thread.cancel()
         if has_canceled(task_id):
             try:
                 exists = await thread_pool_exec(