[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/lib.rs

@@ -34,9 +34,11 @@ pub use types::LoadThreadHistoryParams;
 pub use types::ReadThreadByRolloutPathParams;
 pub use types::ReadThreadParams;
 pub use types::ResumeThreadParams;
+pub use types::SearchThreadsParams;
 pub use types::SortDirection;
 pub use types::StoredThread;
 pub use types::StoredThreadHistory;
+pub use types::StoredThreadSearchResult;
 pub use types::StoredTurn;
 pub use types::StoredTurnError;
 pub use types::StoredTurnItemsView;
@@ -45,6 +47,7 @@ pub use types::ThreadEventPersistenceMode;
 pub use types::ThreadMetadataPatch;
 pub use types::ThreadPage;
 pub use types::ThreadPersistenceMetadata;
+pub use types::ThreadSearchPage;
 pub use types::ThreadSortKey;
 pub use types::TurnPage;
 pub use types::UpdateThreadMetadataParams;