python-telegram-bot's BadRequest inherits from NetworkError, so the
send() retry loop was catching 'Message thread not found' as a transient
network error and retrying 3 times before silently failing. This killed
all tool progress messages, streaming responses, and typing indicators
when the incoming message carried an invalid message_thread_id.
Now detect BadRequest inside the NetworkError handler:
- 'thread not found' + thread_id set → clear thread_id and retry once
(message still reaches the chat, just without topic threading)
- Other BadRequest errors → raise immediately (permanent, don't retry)
- True NetworkError → retry as before (transient)
252 silent failures in gateway.log traced to this on 2026-03-26.
5 new tests for thread fallback, non-thread BadRequest, no-thread sends,
network retry, and multi-chunk fallback.
