31e59fe44d
Bot API 10.1 sendRichMessage treats a lone newline as a soft break, so multi-line content joined with "\n".join(lines) — slash-command lists, etc. — collapses into a single paragraph. Normalize single newlines to Markdown hard breaks (two trailing spaces) in _rich_message_payload, leaving paragraph breaks and fenced code blocks untouched. Fixes #46070