Add occurrence search for paginated threads (#33907)

## What changed

- Add the experimental `thread/searchOccurrences` app-server method for case-insensitive literal search across visible user messages and final assistant messages without replaying the thread.
- Return paginated snippets with UTF-16 match ranges and `thread/turns/list` cursors for navigating to each matching turn.
- Index persisted item types to efficiently select searchable messages from paginated thread history.

## Testing

- Add an app-server integration test covering pagination, steering messages, final-answer selection, Markdown normalization, and UTF-16 offsets.

GitOrigin-RevId: e80160e0f83eb4e5db8de862307085093bee362b

codex-rs/app-server-protocol/src/protocol/common.rs

@@ -630,6 +630,13 @@ client_request_definitions! {
         serialization: None,
         response: v2::ThreadSearchResponse,
     },
+    #[experimental("thread/searchOccurrences")]
+    ThreadSearchOccurrences => "thread/searchOccurrences" {
+        params: v2::ThreadSearchOccurrencesParams,
+        // Explicitly concurrent: this reads persisted paginated history.
+        serialization: None,
+        response: v2::ThreadSearchOccurrencesResponse,
+    },
     ThreadLoadedList => "thread/loaded/list" {
         params: v2::ThreadLoadedListParams,
         serialization: None,