core: add extra metadata field to Thread struct (#29675)

# Summary

Adds a field Thread.extras that can be used to hold arbitrary metadata
specific to a given thread.

codex-rs/app-server-protocol/src/protocol/v2/tests.rs

@@ -168,6 +168,7 @@ fn thread_resume_response_round_trips_initial_turns_page() {
     let response = ThreadResumeResponse {
         thread: Thread {
             id: "thr_123".to_string(),
+            extra: None,
             session_id: "thr_123".to_string(),
             forked_from_id: None,
             parent_thread_id: None,