Cherry-picked from #3311 (stutiredboy). Streaming edits called
edit_message_text(text=buf.text) without chunking, so once accumulated
deltas crossed Telegram's 4096-char limit an ongoing stream would fail
with BadRequest.
Extracts _flush_stream_overflow helper that edits the first chunk in
place, sends any middle chunks, and re-anchors the buffer to a new
message for the tail so subsequent deltas keep streaming.
Co-Authored-By: stutiredboy <stutiredboy@users.noreply.github.com>