bag_of_holding: move persistence to async

[nicoschmdt]

as discussed in #4180 (comment)

[github-actions]

Automated PR Review

0. Summary

• Verdict: LGTM

Moves bag_of_holding persistence off the event loop: replaces the call_later + SimpleNamespace handle pattern with an asyncio.Event-driven background flusher, offloads the actual disk write via asyncio.to_thread, and guards mutation/serialization with db_lock and the temp-file write with disk_lock. Shutdown drains the flusher between writes and performs one final flush. The tricky invariants (clearing the dirty flag with the snapshot; not cancelling mid-thread-write) are called out in comments in the diff.

No further comments, nice job 👍

Generated by PR Review Bot. This is advisory, a human reviewer must still approve.

[patrickelectric]

just to be sure.. how are we supposed to know that the write operation failed ?

[patrickelectric]

The pint here is that: It failed to write in disk, we still return success.

[patrickelectric]

We should put the write_db logic inside flush, to raise any issues when writing to disk.