Summary

Fixes missing markdown → mrkdwn conversion in edit_message(), causing broken formatting and malformed URLs when editing messages.

Problem

The send() method in the Slack adapter correctly converts standard markdown to Slack's mrkdwn format using format_message(). However, edit_message() was sending raw content directly to chat_update, skipping this conversion.

Impact

• Bold text: **text** rendered literally instead of text
• Links with formatting: [link](https://...) in bold/context became <https://...|link>** with trailing ** included in the clickable URL → 404 errors
• Headers/code blocks: Rendered with raw markdown syntax visible

This was discovered after PR #5558 links were unclickable due to trailing ** from bold syntax becoming part of the URL.

Solution

Added format_message(content) call in edit_message() before sending to chat_update, matching the behavior of send().

# Convert standard markdown → Slack mrkdwn
formatted = self.format_message(content)

await self._get_client(chat_id).chat_update(
    channel=chat_id,
    ts=message_id,
    text=formatted,  # Was: text=content
)

Changes

• gateway/platforms/slack.py: Added format_message() call in edit_message()

Testing

All 69 Slack gateway tests pass. The existing test suite validates the format_message() function; this fix ensures edit_message() uses it consistently with send().

Backwards Compatibility

Fully backwards compatible — only fixes broken behavior. Edited messages will now render correctly instead of showing raw markdown.