## What changed
- Add `PersistContext` to the thread-store persistence contract so stores can
identify persistence requested immediately before model sampling. A
`TurnStart` request may be enqueued in the background when later flush and
shutdown operations fence it and report any failure; all other persistence
remains synchronous.
- Use the turn-start context only for the initial input of a regular turn, while
admission acknowledgments, steered input, and other persistence paths keep
the standard durability behavior.
- Flush pending metadata before turn-start persistence and ensure shutdown is
still attempted if that metadata update fails.
## Testing
- Verify that developer instructions and user input are persisted before the
model request completes for a default paginated-history thread.
GitOrigin-RevId: 162f0ec796a61aebe66ca0b909fbcccec3047b85
722784e936