Regular Telegram groups (without Topics/Forums enabled) can send
message_thread_id when users reply to messages. This was incorrectly
being used to create separate session keys like '-123:topic:42',
causing each reply chain to get its own conversation context.
Now resolveTelegramForumThreadId only returns a thread ID when the
chat is actually a forum (is_forum=true). For regular groups, the
thread ID is ignored, ensuring all messages share the same session.
DMs continue to use messageThreadId for thread sessions as before.
