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/common.rs

@@ -2573,6 +2573,7 @@ mod tests {
             response: v2::ThreadStartResponse {
                 thread: v2::Thread {
                     id: "67e55044-10b1-426f-9247-bb680e5fe0c8".to_string(),
+                    extra: None,
                     session_id: "67e55044-10b1-426f-9247-bb680e5fe0c7".to_string(),
                     forked_from_id: None,
                     parent_thread_id: None,
@@ -2622,6 +2623,7 @@ mod tests {
                 "response": {
                     "thread": {
                         "id": "67e55044-10b1-426f-9247-bb680e5fe0c8",
+                        "extra": null,
                         "sessionId": "67e55044-10b1-426f-9247-bb680e5fe0c7",
                         "forkedFromId": null,
                         "parentThreadId": null,