Fix: /file2document/convert blocks event loop on large folders causing 504 timeout (#13784)

Problem

The /file2document/convert endpoint ran all file lookups, document
deletions, and insertions synchronously inside the
request cycle. Linking a large folder (~1.7GB with many files) caused
504 Gateway Timeout because the blocking DB loop
  held the HTTP connection open for too long.

  Fix

- Extracted the heavy DB work into a plain sync function _convert_files
- Inputs are validated and folder file IDs expanded upfront (fast path)
- The blocking work is dispatched to a thread pool via
get_running_loop().run_in_executor() and the endpoint returns 200
  immediately
- Frontend only checks data.code === 0 so the response change
(file2documents list → True) has no impact

  Fixes #13781

---------

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

test/testcases/test_web_api/test_user_app/test_user_app_unit.py

@@ -230,7 +230,7 @@ def _load_user_app(monkeypatch):
             return True
 
         @staticmethod
-        def get_api_key(tenant_id, model_name):
+        def get_api_key(tenant_id, model_name, model_type=None):
             return _MockTableObject(
                 id=1,
                 tenant_id=tenant_id,