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_fork.rs

@@ -18,6 +18,7 @@ use codex_app_server_protocol::ThreadStatus;
 use codex_app_server_protocol::TurnStatus;
 use codex_app_server_protocol::UserInput;
 use pretty_assertions::assert_eq;
+use serde_json::Value;
 use std::path::Path;
 use tempfile::TempDir;
 use tokio::time::timeout;
@@ -70,8 +71,20 @@ async fn thread_fork_creates_new_thread_and_emits_started() -> Result<()> {
         mcp.read_stream_until_response_message(RequestId::Integer(fork_id)),
     )
     .await??;
+    let fork_result = fork_resp.result.clone();
     let ThreadForkResponse { thread, .. } = to_response::<ThreadForkResponse>(fork_resp)?;
 
+    // Wire contract: thread title field is `name`, serialized as null when unset.
+    let thread_json = fork_result
+        .get("thread")
+        .and_then(Value::as_object)
+        .expect("thread/fork result.thread must be an object");
+    assert_eq!(
+        thread_json.get("name"),
+        Some(&Value::Null),
+        "forked threads do not inherit a name; expected `name: null`"
+    );
+
     let after_contents = std::fs::read_to_string(&original_path)?;
     assert_eq!(
         after_contents, original_contents,
@@ -87,6 +100,7 @@ async fn thread_fork_creates_new_thread_and_emits_started() -> Result<()> {
     assert_ne!(thread_path, original_path);
     assert!(thread.cwd.is_absolute());
     assert_eq!(thread.source, SessionSource::VsCode);
+    assert_eq!(thread.name, None);
 
     assert_eq!(
         thread.turns.len(),
@@ -115,6 +129,16 @@ async fn thread_fork_creates_new_thread_and_emits_started() -> Result<()> {
         mcp.read_stream_until_notification_message("thread/started"),
     )
     .await??;
+    let started_params = notif.params.clone().expect("params must be present");
+    let started_thread_json = started_params
+        .get("thread")
+        .and_then(Value::as_object)
+        .expect("thread/started params.thread must be an object");
+    assert_eq!(
+        started_thread_json.get("name"),
+        Some(&Value::Null),
+        "thread/started must serialize `name: null` when unset"
+    );
     let started: ThreadStartedNotification =
         serde_json::from_value(notif.params.expect("params must be present"))?;
     assert_eq!(started.thread, thread);