Record realtime close marker on replacement (#13058)

## Summary
- record a realtime close developer message when a new realtime session
replaces an active one
- assert the replacement marker through the mocked responses request
path

---------

Co-authored-by: Codex <noreply@openai.com>
Co-authored-by: Charles Cunningham <ccunningham@openai.com>

codex-rs/core/tests/suite/realtime_conversation.rs

@@ -367,6 +367,7 @@ async fn conversation_second_start_replaces_runtime() -> Result<()> {
     server.shutdown().await;
     Ok(())
 }
+
 #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
 async fn conversation_uses_experimental_realtime_ws_base_url_override() -> Result<()> {
     skip_if_no_network!(Ok(()));