[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/app-server/src/message_processor.rs

@@ -1072,6 +1072,9 @@ impl MessageProcessor {
             ClientRequest::ThreadList { params, .. } => {
                 self.thread_processor.thread_list(params).await
             }
+            ClientRequest::ThreadSearch { params, .. } => {
+                self.thread_processor.thread_search(params).await
+            }
             ClientRequest::ThreadLoadedList { params, .. } => {
                 self.thread_processor.thread_loaded_list(params).await
             }