Allow custom Codex instructions for realtime transitions (#36408)

## What changed

- Add optional `realtimeStartInstructions` and `realtimeEndInstructions` fields to `thread/realtime/start`.
- Apply the instructions when Codex enters and leaves realtime mode, while preserving the existing defaults when either field is omitted.
- Limit each field to 8,192 estimated tokens.

## Testing

- Cover custom entry and exit instructions across realtime state transitions.
- Cover token-limit validation for both fields.

GitOrigin-RevId: b9cfa1ff22e084d8a6b2ed56bd27d95ffdd6b6bd

codex-rs/core/src/session/tests.rs

@@ -8808,7 +8808,7 @@ async fn record_context_updates_emits_realtime_end_when_session_stops_being_live
     assert!(
         developer_texts
             .iter()
-            .any(|text| text.contains("Reason: inactive")),
+            .any(|text| text.contains("<realtime_conversation>")),
         "expected a realtime end update, got {developer_texts:?}"
     );
 }

codex-rs/core/src/session/world_state.rs

@@ -96,12 +96,19 @@ impl Session {
         {
             world_state.add_section(ContextWindowGuidanceState::new(guidance));
         }
+        let realtime_mode_instructions = self.conversation.mode_instructions().await;
         world_state.add_section(RealtimeState::new(
             turn_context.realtime_active,
-            turn_context
-                .config
-                .experimental_realtime_start_instructions
-                .as_deref(),
+            realtime_mode_instructions
+                .as_ref()
+                .and_then(|instructions| instructions.start.as_deref())
+                .or(turn_context
+                    .config
+                    .experimental_realtime_start_instructions
+                    .as_deref()),
+            realtime_mode_instructions
+                .as_ref()
+                .and_then(|instructions| instructions.end.as_deref()),
         ));
         world_state.add_section(AgentsMdState::new(step_context.loaded_agents_md.as_deref()));
         if turn_context.config.include_permissions_instructions {