fix: TTL management task no longer crashes on missing file records

[jcaille]

Description

The TTL cleanup task would crash with a RuntimeError when trying to delete a file record that no longer exists, blocking cleanup of all remaining sessions.

2026-03-30 18:31:16.948infoTraceback (most recent call last):
  File "/usr/local/lib/python3.11/site-packages/celery/app/trace.py", line 453, in trace_task
    R = retval = fun(*args, **kwargs)
                 ^^^^^^^^^^^^^^^^^^^^
  File "/app/onyx/background/celery/apps/app_base.py", line 87, in __call__
    return super().__call__(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/celery/app/trace.py", line 736, in __protected_call__
    return self.run(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/app/ee/onyx/background/celery/tasks/ttl_management/tasks.py", line 60, in perform_ttl_management_task
    delete_chat_session(
  File "/app/onyx/db/chat.py", line 322, in delete_chat_session
    delete_messages_and_files_from_chat_session(chat_session_id, db_session)
  File "/app/onyx/db/chat.py", line 169, in delete_messages_and_files_from_chat_session
    file_store.delete_file(file_id=file_info.get("id"))
  File "/app/onyx/file_store/file_store.py", line 459, in delete_file
    file_record = get_filerecord_by_file_id(
                  ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/app/onyx/db/file_record.py", line 42, in get_filerecord_by_file_id
    raise RuntimeError(f"File by id {file_id} does not exist or was deleted")

Two fixes proposed:

1. Wrap file deletion in delete_messages_and_files_from_chat_session with try/except — a missing file is already the desired state, so log a warning and continue.

2. Add per-session error handling in the TTL task loop. The existing comment said "one session per delete so that we don't blow up" but the outer try/except still aborted on the first failure. Now each session deletion is individually wrapped so failures don't block cleanup of subsequent sessions.

Additional Options

• [Optional] Please cherry-pick this PR to the latest release version.
• [Optional] Override Linear Check

---

Summary by cubic

Prevents TTL cleanup from crashing on missing file records by skipping those deletes and isolating per-session errors. Cleanup continues for all sessions and the task reports failure only if any deletion fails.

• Bug Fixes

  • In delete_messages_and_files_from_chat_session, wrap file deletes in try/except; log a warning and continue on missing files.
  • In perform_ttl_management_task, handle each session in try/except, count failures, and set success to failures == 0.
  • Added unit tests for chat deletion and TTL task resilience.

• Refactors

  • Aligned test conventions (helper extraction, unused imports removed, consistent naming).

^{Written for commit 569fef2. Summary will update on new commits.}