Feishu already had text batching with a static 0.6s delay. This adds
adaptive delay: waits 2.0s when a chunk is near the ~4096-char split
point since a continuation is almost certain.
Tracks _last_chunk_len on each queued event to determine the delay.
Configurable via HERMES_FEISHU_TEXT_BATCH_SPLIT_DELAY_SECONDS (default 2.0).
Ref #6892