Telegram: support guest mode (guest_message updates and answerGuestQuery) #871

Bot API 10.0 (May 2026) added guest mode: users can @-mention a bot in chats it is not a member of. The update arrives as the new guest_message field on Update, with guest_bot_caller_user, guest_bot_caller_chat and guest_query_id on the message, and replies must go through the new answerGuestQuery method (returning SentGuestMessage) — sendMessage does not work for these.

The Telegram adapter's processUpdate currently only inspects message / edited_message / channel_post / edited_channel_post / callback_query / message_reaction, so guest updates are dropped silently and the bot appears unresponsive to guest callers.

Needed:

• parse update.guest_message and dispatch it with the guest context attached (a mention-like event seems natural, since guest calls are always @-mentions)
• a reply path using answerGuestQuery with the guest_query_id
• a docs note that existing webhooks may need guest_message in allowed_updates

Refs: Bot API changelog, Telegram announcement