72da4f8657
Slack's chat.update enforces the same ~40k character limit as chat.postMessage, but edit_message sent the formatted text unchunked — an oversized edit failed outright with msg_too_long and the message was never updated. Unlike send() we cannot split an edit into multiple messages, so truncate to MAX_MESSAGE_LENGTH via the shared chunker (which preserves code-block boundaries) and keep the first chunk. Reapplied from PR #33224 (targeted the pre-plugin gateway/platforms/slack.py path) onto plugins/platforms/slack/adapter.py.