Load model context from a bounded rollout suffix (#32896)

## Why

Reconstructing the latest model-visible context does not require replaying an
entire paginated rollout when a usable compaction checkpoint and the associated
completed-turn metadata are available.

## What changed

- Add `ThreadStore::load_latest_model_context` and `StoredModelContext` for
  loading replay-ready model context independently of full thread history.
- Reverse-scan plain paginated JSONL rollouts until the newest safe bounded
  suffix is found, while preserving canonical session metadata and chronological
  replay order.
- Fall back to complete history for legacy or compressed rollouts and whenever
  compaction or rollback records make a bounded cutoff unsafe.

## Testing

- Cover checkpoint selection, turn-metadata boundaries, agent messages,
  contextual user fragments, and full-history fallbacks.

GitOrigin-RevId: 3572f4ecc7aa4099a6d9f0d3e72d0ef432cd9497

codex-rs/thread-store/src/lib.rs

@@ -39,6 +39,7 @@ pub use types::ReadThreadParams;
 pub use types::ResumeThreadParams;
 pub use types::SearchThreadsParams;
 pub use types::SortDirection;
+pub use types::StoredModelContext;
 pub use types::StoredThread;
 pub use types::StoredThreadHistory;
 pub use types::StoredThreadItem;