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>

api/db/services/file_service.py

@@ -561,14 +561,9 @@ class FileService(CommonService):
 
     @staticmethod
     def parse_docs(file_objs, user_id):
-        exe = ThreadPoolExecutor(max_workers=12)
-        threads = []
-        for file in file_objs:
-            threads.append(exe.submit(FileService.parse, file.filename, file.read(), False))
-
-        res = []
-        for th in threads:
-            res.append(th.result())
+        with ThreadPoolExecutor(max_workers=12) as exe:
+            threads = [exe.submit(FileService.parse, file.filename, file.read(), False) for file in file_objs]
+            res = [th.result() for th in threads]
 
         return "\n\n".join(res)
 
@@ -793,19 +788,21 @@ class FileService(CommonService):
         def image_to_base64(file):
             return "data:{};base64,{}".format(file["mime_type"],
                                         base64.b64encode(FileService.get_blob(file["created_by"], file["id"])).decode("utf-8"))
-        exe = ThreadPoolExecutor(max_workers=5)
         threads = []
         imgs = []
-        for file in files:
-            if file["mime_type"].find("image") >=0:
-                if raw:
-                    imgs.append(FileService.get_blob(file["created_by"], file["id"]))
-                else:
-                    threads.append(exe.submit(image_to_base64, file))
-                continue
-            threads.append(exe.submit(FileService.parse, file["name"], FileService.get_blob(file["created_by"], file["id"]), True, file["created_by"], layout_recognize))
-    
+        with ThreadPoolExecutor(max_workers=5) as exe:
+            for file in files:
+                if file["mime_type"].find("image") >=0:
+                    if raw:
+                        imgs.append(FileService.get_blob(file["created_by"], file["id"]))
+                    else:
+                        threads.append(exe.submit(image_to_base64, file))
+                    continue
+                threads.append(exe.submit(FileService.parse, file["name"], FileService.get_blob(file["created_by"], file["id"]), True, file["created_by"], layout_recognize))
+
+            results = [th.result() for th in threads]
+
         if raw:
-            return [th.result() for th in threads], imgs
+            return results, imgs
         else:
-            return [th.result() for th in threads]
+            return results