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/tests/suite/v2/thread_unarchive.rs

@@ -18,6 +18,7 @@ use codex_app_server_protocol::UserInput;
 use codex_core::find_archived_thread_path_by_id_str;
 use codex_core::find_thread_path_by_id_str;
 use pretty_assertions::assert_eq;
+use serde_json::Value;
 use std::fs::FileTimes;
 use std::fs::OpenOptions;
 use std::path::Path;
@@ -120,6 +121,7 @@ async fn thread_unarchive_moves_rollout_back_into_sessions_directory() -> Result
         mcp.read_stream_until_response_message(RequestId::Integer(unarchive_id)),
     )
     .await??;
+    let unarchive_result = unarchive_resp.result.clone();
     let ThreadUnarchiveResponse {
         thread: unarchived_thread,
     } = to_response::<ThreadUnarchiveResponse>(unarchive_resp)?;
@@ -140,6 +142,18 @@ async fn thread_unarchive_moves_rollout_back_into_sessions_directory() -> Result
     );
     assert_eq!(unarchived_thread.status, ThreadStatus::NotLoaded);
 
+    // Wire contract: thread title field is `name`, serialized as null when unset.
+    let thread_json = unarchive_result
+        .get("thread")
+        .and_then(Value::as_object)
+        .expect("thread/unarchive result.thread must be an object");
+    assert_eq!(unarchived_thread.name, None);
+    assert_eq!(
+        thread_json.get("name"),
+        Some(&Value::Null),
+        "thread/unarchive must serialize `name: null` when unset"
+    );
+
     let rollout_path_display = rollout_path.display();
     assert!(
         rollout_path.exists(),