[codex] Add rollout-backed thread content search (#23519)

## Summary
- add experimental `thread/search` for local rollout-backed thread
search using `rg` over JSONL rollouts
- return search-specific result rows with optional previews instead of
storing preview data on `StoredThread` or ordinary `Thread` responses
- keep `thread/list` separate from full-content search and document the
new app-server surface

## Testing
- `cargo test -p codex-app-server-protocol`
- `cargo test -p codex-app-server
thread_search_returns_content_and_title_matches -- --nocapture`

codex-rs/thread-store/src/local/list_threads.rs

@@ -107,7 +107,7 @@ pub(super) async fn list_threads(
     Ok(ThreadPage { items, next_cursor })
 }
 
-async fn list_rollout_threads(
+pub(super) async fn list_rollout_threads(
     state_db: Option<codex_rollout::StateDbHandle>,
     config: &RolloutConfig,
     default_model_provider_id: &str,