fix(gateway): Apply markdown-to-mrkdwn conversion in edit_message

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.

gateway/platforms/slack.py

@@ -276,10 +276,13 @@ class SlackAdapter(BasePlatformAdapter):
         if not self._app:
             return SendResult(success=False, error="Not connected")
         try:
+            # 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=content,
+                text=formatted,
             )
             return SendResult(success=True, message_id=message_id)
         except Exception as e:  # pragma: no cover - defensive logging