The edit_message method was sending raw content directly to Slack's
chat_update API without converting standard markdown to Slack's mrkdwn
format. This caused broken formatting and malformed URLs (e.g., trailing
** from bold syntax became part of clickable links → 404 errors).
The send() method already calls format_message() to handle this conversion,
but edit_message() was bypassing it. This change ensures edited messages
receive the same markdown → mrkdwn transformation as new messages.
Closes: PR #5558 formatting issue where links had trailing markdown syntax.
9b6e5f6a04