fix(qq): resolve WebSocket disconnection and session resume issues

- Add heartbeat ACK timeout detection (MAX_MISSED_ACKS=3) to detect zombie connections
- Implement session resume support with persistent session_id/last_sequence
- Add exponential backoff reconnection (1s-60s) to avoid rate limiting
- Optimize message deduplication (HashSet 10k capacity with half-eviction)
- Add dynamic heartbeat interval from server Hello + 10% grace period
- Add 9 new unit tests for heartbeat, dedup, and retry logic

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Author: mastwet