Add field to Thread object for the latest rename set for a given thread (#12301)

Exposes through the app server updated names set for a thread. This
enables other surfaces to use the core as the source of truth for thread
naming. `threadName` is gathered using the helper functions used to
interact with `session_index.jsonl`, and is hydrated in:
- `thread/list`
- `thread/read`
- `thread/resume`
- `thread/unarchive`
- `thread/rollback`

We don't do this for `thread/start` and `thread/fork`.

codex-rs/app-server/src/thread_status.rs

@@ -626,6 +626,7 @@ mod tests {
             agent_role: None,
             source,
             git_info: None,
+            name: None,
             turns: Vec::new(),
         }
     }