fix: keep a reference to the throttled cleanup background task (avoid GC before completion)

[kratos0718]

What

openai_create_vector_store_file_batch schedules the throttled expired-batch cleanup with a bare asyncio.create_task(...) whose result is discarded:

asyncio.create_task(self._cleanup_expired_file_batches())

The event loop keeps only a weak reference to a task, so a fire-and-forget task can be garbage-collected before it finishes — silently dropping the cleanup mid-run. This is the pattern behind Ruff's RUF006 and the asyncio docs warning.

Notably, the file-batch task a few lines above is already handled correctly (self._file_batch_tasks[batch_id] = task) — this cleanup task just slipped through.

Fix

Keep a strong reference via a dedicated self._background_tasks set with a self-cleaning done-callback (the standard pattern), and cancel those tasks in shutdown() alongside the existing file-batch tasks — matching the lifecycle handling already present in this mixin.

Notes

• Behavior-preserving: the cleanup runs exactly as before; it just can't be GC'd mid-flight.
• Minimal diff, no new dependencies.