Add app-server support for reverting paginated threads - #38440

## What changed

- Add the experimental `thread/revert` request, which replaces a loaded paginated thread's durable history with the prefix before `beforeTurnId` while preserving the thread ID.
- Interrupt any active turn, reload the replacement history without unloading the thread, preserve mutable thread settings, and emit `thread/reverted`.
- Return backward pagination cursors for the retained turns and items, reject stale rollout paths after a revert, and document that local file changes are unaffected.

## Testing

- Cover history replacement, pagination, stale paths, missing turns, subsequent turns, active-turn interruption, and preserved thread settings.

GitOrigin-RevId: 435a8163ad4f310cc47219b436f6bdde21f2b83a