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/tui/src/lib.rs

@@ -1280,6 +1280,7 @@ mod tests {
             model,
             personality: None,
             collaboration_mode: None,
+            realtime_active: Some(false),
             effort: config.model_reasoning_effort,
             summary: config
                 .model_reasoning_summary