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/common/mcp_process.rs

@@ -59,6 +59,7 @@ use codex_app_server_protocol::ThreadLoadedListParams;
 use codex_app_server_protocol::ThreadReadParams;
 use codex_app_server_protocol::ThreadResumeParams;
 use codex_app_server_protocol::ThreadRollbackParams;
+use codex_app_server_protocol::ThreadSetNameParams;
 use codex_app_server_protocol::ThreadStartParams;
 use codex_app_server_protocol::ThreadUnarchiveParams;
 use codex_app_server_protocol::TurnCompletedNotification;
@@ -418,6 +419,15 @@ impl McpProcess {
         self.send_request("thread/archive", params).await
     }
 
+    /// Send a `thread/name/set` JSON-RPC request.
+    pub async fn send_thread_set_name_request(
+        &mut self,
+        params: ThreadSetNameParams,
+    ) -> anyhow::Result<i64> {
+        let params = Some(serde_json::to_value(params)?);
+        self.send_request("thread/name/set", params).await
+    }
+
     /// Send a `thread/unarchive` JSON-RPC request.
     pub async fn send_thread_unarchive_request(
         &mut self,