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/protocol/src/protocol.rs

@@ -70,6 +70,8 @@ pub const ENVIRONMENT_CONTEXT_OPEN_TAG: &str = "<environment_context>";
 pub const ENVIRONMENT_CONTEXT_CLOSE_TAG: &str = "</environment_context>";
 pub const COLLABORATION_MODE_OPEN_TAG: &str = "<collaboration_mode>";
 pub const COLLABORATION_MODE_CLOSE_TAG: &str = "</collaboration_mode>";
+pub const REALTIME_CONVERSATION_OPEN_TAG: &str = "<realtime_conversation>";
+pub const REALTIME_CONVERSATION_CLOSE_TAG: &str = "</realtime_conversation>";
 pub const USER_MESSAGE_BEGIN: &str = "## My request for Codex:";
 
 /// Submission Queue Entry - requests from user
@@ -2146,6 +2148,8 @@ pub struct TurnContextItem {
     pub personality: Option<Personality>,
     #[serde(default, skip_serializing_if = "Option::is_none")]
     pub collaboration_mode: Option<CollaborationMode>,
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub realtime_active: Option<bool>,
     #[serde(skip_serializing_if = "Option::is_none")]
     pub effort: Option<ReasoningEffortConfig>,
     pub summary: ReasoningSummaryConfig,
@@ -3376,6 +3380,7 @@ mod tests {
             model: "gpt-5".to_string(),
             personality: None,
             collaboration_mode: None,
+            realtime_active: None,
             effort: None,
             summary: ReasoningSummaryConfig::Auto,
             user_instructions: None,