send_slash_confirm() sent the raw command preview with ParseMode.MARKDOWN,
skipping the format_message() conversion applied to every other dynamic
send in the adapter. Commands with underscores, dots, brackets, or other
MarkdownV2-sensitive characters raised BadRequest: Can't parse entities;
the exception was swallowed by the outer try/except, so the confirmation
prompt silently never appeared.
Fix: wrap preview through format_message() and switch to MARKDOWN_V2,
symmetric with send_update_prompt and the callback sends fixed in
a69404052.
