When sending multi-chunk responses, individual chunks can fail due to
transient iLink API errors. Previously a single failure would abort the
entire message. Now each chunk is retried with linear backoff before
giving up, and the same client_id is reused across retries for
server-side deduplication.
Configurable via config.yaml (platforms.weixin.extra) or env vars:
- send_chunk_delay_seconds (default 0.35s) — pacing between chunks
- send_chunk_retries (default 2) — max retry attempts per chunk
- send_chunk_retry_delay_seconds (default 1.0s) — base retry delay
Replaces the hardcoded 0.3s inter-chunk delay from #7903.
Salvaged from PR #7899 by @corazzione. Fixes#7836.
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