[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-protocol/src/protocol/common.rs

@@ -569,6 +569,12 @@ client_request_definitions! {
         serialization: None,
         response: v2::ThreadListResponse,
     },
+    #[experimental("thread/search")]
+    ThreadSearch => "thread/search" {
+        params: v2::ThreadSearchParams,
+        serialization: None,
+        response: v2::ThreadSearchResponse,
+    },
     ThreadLoadedList => "thread/loaded/list" {
         params: v2::ThreadLoadedListParams,
         serialization: None,