The overflow split loop required _message_id to be set, but on the
first streamed message (or after a segment break) _message_id is None.
Oversized text fell through to _send_or_edit → adapter.send(), which
split internally — but subsequent edits hit Telegram's 'message too
long' and were silently truncated with '…', cutting off the response.
Add a new code path for the _message_id is None case that uses
truncate_message() (same as the non-streaming path) to split with
proper word/code-fence boundaries and chunk indicators. Each chunk
is sent as a new message via _send_new_chunk().
Properly handles got_done (returns immediately after sending chunks
instead of continuing into an infinite loop) and got_segment_break.
Original cherry-picked from PR #6816 by dangelo352.
Fixes silent message truncation on Telegram for long streamed responses.
aed9b90ae3