Escape tags in rich replies as HTML entities

Escape tags in rich replies as HTML entities

The bot writes each < of a reply outside inline code and fenced code
blocks as &lt; and each & as &amp; before it sends the reply with
sendRichMessage. Rich Markdown does not read a backslash before < as an
escape: it shows the backslash and still reads the tag as HTML. It reads
HTML entities, so &lt; shows as <. The escape of & makes an entity that
the model writes in prose, such as &lt;, show as written.

The entity fallback reads no HTML, so a rich part that Telegram refuses
goes to the fallback with the escape reversed, and the user sees the
text as the model wrote it. The reverse is exact, because after the
escape each & outside code starts an entity of the escape. The escape
and its reverse find the code of a text in the same way, so the reverse
also works on each part of the split.

The split still counts the escaped text against the rich limit, and the
fallback splits the text without the escape at its own limit. When the
split cuts a line longer than the limit, it cuts before an & in the last
four bytes, so that it does not cut an entity.

Issue: d75e312