test(run-engine): add integration tests for V2/V3 message format handling

Add comprehensive tests covering:
- V2 (legacy) format enqueue/dequeue/ack/nack
- V3 (optimized) format enqueue/dequeue/ack/nack
- Mixed format migration scenarios
- Format detection helpers

https://claude.ai/code/session_01AyzQp6tbj7th5QRTCYjJR5

Author: claude