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/local/mod.rs

@@ -4,6 +4,7 @@ mod delete_thread;
 mod helpers;
 mod list_threads;
 mod live_writer;
+mod model_context;
 mod read_thread;
 mod search_threads;
 mod unarchive_thread;
@@ -32,6 +33,7 @@ use crate::ReadThreadByRolloutPathParams;
 use crate::ReadThreadParams;
 use crate::ResumeThreadParams;
 use crate::SearchThreadsParams;
+use crate::StoredModelContext;
 use crate::StoredThread;
 use crate::StoredThreadHistory;
 use crate::ThreadPage;
@@ -277,6 +279,13 @@ impl ThreadStore for LocalThreadStore {
         Box::pin(LocalThreadStore::load_history(self, params))
     }
 
+    fn load_latest_model_context(
+        &self,
+        params: LoadThreadHistoryParams,
+    ) -> ThreadStoreFuture<'_, StoredModelContext> {
+        Box::pin(async move { model_context::load_latest_model_context(self, params).await })
+    }
+
     fn read_thread(&self, params: ReadThreadParams) -> ThreadStoreFuture<'_, StoredThread> {
         Box::pin(async move { read_thread::read_thread(self, params).await })
     }