The rich branch in send_delta(stream_end=True) was unreachable: it was
guarded by 'not buf.message_id' after an earlier return had already
ensured buf.message_id is set, so sendRichMessage never fired with
streaming enabled and the final message always went through the legacy
HTML editMessageText path.
Bot API 10.1 added a rich_message parameter to editMessageText, which
upgrades an existing message to rich in place. Use it at stream end via
do_api_request so the streaming preview keeps its identity — no
delete-and-resend, so none of the flickering or dropped line breaks
that made rich-at-stream-end fail in #4470.
Capability errors (server older than 10.1) trip the existing
_rich_send_disabled latch and fall back to the legacy HTML path, which
is unchanged.
Fixes#5516
