## What changed
- Resolve the turn ID for restored token usage before sending resume and fork responses, avoiding a clone of the full thread history.
- Add `ThreadHistoryBuilder::active_turn_id` so attribution can inspect the active turn without materializing its items.
- Stop history reconstruction at the latest persisted `TokenCount` and verify that a later tail turn does not receive the replayed usage.
GitOrigin-RevId: 69c4a9fe04770d1a938470afc933431e61670094