[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/store.rs

@@ -13,9 +13,11 @@ use crate::LoadThreadHistoryParams;
 use crate::ReadThreadByRolloutPathParams;
 use crate::ReadThreadParams;
 use crate::ResumeThreadParams;
+use crate::SearchThreadsParams;
 use crate::StoredThread;
 use crate::StoredThreadHistory;
 use crate::ThreadPage;
+use crate::ThreadSearchPage;
 use crate::ThreadStoreError;
 use crate::ThreadStoreResult;
 use crate::TurnPage;
@@ -76,6 +78,16 @@ pub trait ThreadStore: Any + Send + Sync {
     /// Lists stored threads matching the supplied filters.
     async fn list_threads(&self, params: ListThreadsParams) -> ThreadStoreResult<ThreadPage>;
 
+    /// Searches stored threads and returns search-only preview metadata.
+    async fn search_threads(
+        &self,
+        _params: SearchThreadsParams,
+    ) -> ThreadStoreResult<ThreadSearchPage> {
+        Err(ThreadStoreError::Unsupported {
+            operation: "thread/search",
+        })
+    }
+
     /// Lists turns within a stored thread.
     async fn list_turns(&self, _params: ListTurnsParams) -> ThreadStoreResult<TurnPage> {
         Err(ThreadStoreError::Unsupported {