oEmbed discovery fails on XML-encoded link URLs

[cweiske]

Special characters like & in URLs should be encoded in HTML pages (& becomes &).

Wordpress' oEmbed discovery in WP_oEmbed::discover() does not decode the encoded entities and then tries to fetch URLs like

​http://example.org/oembed.php?format=json&url=...

It should use htmlentities_decode() on the extracted URLs.

[wonderboymusic]

In 31413:

oEmbed discovery fails on encoded link URLs: decode HTML chars in the HTML-encoded URLs that are returned.

Props cweiske.
Fixes #31213.