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/state/src/extract.rs

@@ -260,6 +260,7 @@ mod tests {
                 model: "gpt-5".to_string(),
                 personality: None,
                 collaboration_mode: None,
+                realtime_active: None,
                 effort: None,
                 summary: ReasoningSummary::Auto,
                 user_instructions: None,
@@ -296,6 +297,7 @@ mod tests {
                 model: "gpt-5".to_string(),
                 personality: None,
                 collaboration_mode: None,
+                realtime_active: None,
                 effort: None,
                 summary: ReasoningSummary::Auto,
                 user_instructions: None,