Forum supergroups always have chat.type == SUPERGROUP in the Telegram API,
but the General topic fallback in _build_message_event() only checked for
chat_type == "group". While the current mapping happens to map SUPERGROUP
to "group", this is fragile — other methods (e.g. _is_group_chat) already
distinguish the two types, and any refactor to the mapping would silently
break General topic routing.
Extend the check to accept both "group" and "supergroup", and add a debug
log when the fallback fires to aid future diagnosis.
027e054a17