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.
72da4f8657