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/app-server/src/request_processors/turn_processor.rs

@@ -1095,6 +1095,8 @@ impl TurnRequestProcessor {
                         role: item.role,
                     })
                     .collect(),
+                realtime_start_instructions: params.realtime_start_instructions,
+                realtime_end_instructions: params.realtime_end_instructions,
                 prompt: params.prompt,
                 realtime_session_id: params.realtime_session_id,
                 transport: params.transport.map(|transport| match transport {