When a DM topic lane's message_thread_id is rejected by Telegram
(e.g. stale or deleted topic), send_typing now falls back to sending
the typing indicator without thread_id so it at least appears in the
main DM view, rather than being silently swallowed.
Also adds test for the fallback behavior.
When sending messages containing @username patterns, auto-generate
MessageEntity(type='mention') entries so that the receiving bot's
require_mention filter can trigger. This enables proper bot-to-bot
interop where mention-based routing is used.
fonhal