Comments: Rebuild embed players instead of running kses on them

[arcangelini]

Proposed changes

• Fix YouTube embeds showing [youtube …] text instead of a player on WordPress.com, in both the editor preview and the posted comment.
• Cause: since Comments: strip provider scripts from embeds and skip previews in wp-admin #53364 the embed HTML goes through wp_kses(). On WordPress.com, pre_kses reversals turn known players into shortcodes, and nothing expanded them. Those are the Shortcodes module's reversals plus wpcom's Filter_Embedded_HTML_Objects registrations, covering YouTube, Slideshare, SoundCloud, Kickstarter and Flickr.
• Embeds::sanitize_html() no longer runs kses. It reads the provider's HTML with WP_HTML_Tag_Processor and writes the first <iframe> or <img> as a new tag. Only src (https) plus a short list of attributes come along. Scripts, inline handlers, srcdoc and inline styles never get copied.
• Embeds that need a provider script, such as TikTok and Reddit, return nothing, so the editor and the posted comment show a link. Since Comments: strip provider scripts from embeds and skip previews in wp-admin #53364 stripped their scripts, they only rendered as a static blockquote anyway.
• No cleanup needed: posted comments sanitize at render and cached previews sanitize on the way out.

Does this pull request change what data or activity we track or use?

No.

Testing instructions

• On a WordPress.com Simple site with Jetpack Comments on, open a post and start a comment.
• Add an embed with "/" > Embed and a YouTube URL. The preview shows a player, not [youtube …] text. Post it and check the comment shows the player.
• Repeat with Vimeo or Spotify: still a player.
• Try a TikTok URL: it becomes a link.

[github-actions]

Are you an Automattician? Please test your changes on all WordPress.com environments to help mitigate accidental explosions.

• To test on WoA, go to the Plugins menu on a WoA dev site. Click on the "Upload" button and follow the upgrade flow to be able to upload, install, and activate the Jetpack Beta plugin. Once the plugin is active, go to Jetpack > Jetpack Beta, select your plugin (Jetpack or WordPress.com Site Helper), and enable the fix/comments-youtube-embed-shortcode branch.
• To test on Simple, run the following command on your sandbox:

bin/jetpack-downloader test jetpack fix/comments-youtube-embed-shortcode

bin/jetpack-downloader test jetpack-mu-wpcom-plugin fix/comments-youtube-embed-shortcode

Interested in more tips and information?

• In your local development environment, use the jetpack rsync command to sync your changes to a WoA dev blog.
• Read more about our development workflow here: PCYsg-eg0-p2
• Figure out when your changes will be shipped to customers here: PCYsg-eg5-p2

[github-actions]

Thank you for your PR!

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[jp-launch-control]

Code Coverage Summary

This PR did not change code coverage!

That could be good or bad, depending on the situation. Everything covered before, and still is? Great! Nothing was covered before? Not so great. 🤷

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