- Change MAX_MESSAGE_LENGTH from 4000 to 2000 to match Weixin iLink API limit
- Add RATE_LIMIT_ERRCODE = -2 handling with 3x backoff retry
- Increase default send_chunk_delay_seconds from 0.35 to 1.5 to avoid rate limits
- Increase default send_chunk_retries from 2 to 4 for better reliability
- Use _split_text() in send() to chunk long messages before delivery
Fixes#16411