Shortcode: Fix the YouTube URL for the private video

[arthur791004]

Fixes p1725499365595119-slack-CRWCHQGUB

Proposed changes:

• There is a regression caused by Shortcode: Get rid of custom shortcodes for YouTube and Vimeo in favor of Core #39096 as we didn't notice that the custom embed handler would be called by the /oembed/1.0/sites/:site_id/proxy endpoint. Before that change, we could always get the result from the oembed proxy endpoint for every YouTube URL even if it was either private or invalid. As a result, the scheduled YouTube video (as a private video) could be embedded correctly.
• To make the scheduled YouTube video work as before, we reverted part of that PR and made a slight change to keep the title of the YouTube video available on the iframe. But it also means that every YouTube URL can be embedded correctly even if it's the wrong one.
• The changes might look weird as the custom embed hanlder ($wp_embed->get_embed_handler_html) works differently between the oembed proxy endpoint and how it renders on the frontend.
  • For the oembed proxy endpoint, the order is
    • Get the data from the cache
    • Get the data from the oembed endpoint according to the provider of the URL. For YouTube, it's https://www.youtube.com/oembed?format=json&url=
    • If we cannot get the result, fallback to the custom oembed handler, that is, wpcom_youtube_embed_crazy_url
    • See here for more details.
  • For how it renders on the frontend, the order is
    • Get the HTML from the custom oembed handler first
    • If not, get the HTML from the cache
    • If not, get the HTML from the oembed endpoint according to the provider of the URL
    • See here for more details
• As mentioned above, the HTML from the custom handler is different. If we keep the previous way, the title of the YouTube video would be missing because the custom embed handler of YouTube couldn't get the title of the video. Hence, this PR registers the custom embed handler of YouTube for the oembed proxy endpoint and then hooks the HTML from the custom handler to the pre_oembed_result filter to deprioritize its order on the frontend
• Maybe we have to raise the PR to Core to make the order the same in the follow-up PR 🤔

Other information:

• Have you written new tests for your changes, if applicable?
• Have you checked the E2E test CI results, and verified that your changes do not break them?
• Have you tested your changes on WordPress.com, if applicable (if so, you'll see a generated comment below with a script to run)?

Jetpack product discussion

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

Testing instructions:

• Apply changes to wpcom
• Sandbox your site
• Go to create a new post
• Embed the following YouTube URLs
  • https://www.youtube.com/watch?v=63XUTCLOCnM (Public)
  • https://www.youtube.com/watch?v=63XUTCLOCnMxxx (Invalid)
  • https://www.youtube.com/watch?v=X47UjFQXxAY (Scheduled Private Video)
• Make sure all of the above videos can be embedded correctly
• Make sure there is the title attribute on the iframe of the public video (the first one)

[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 WordPress.com Simple 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, and enable the fix/youtube-schedule-video branch.

• To test on Simple, run the following command on your sandbox:

  bin/jetpack-downloader test jetpack fix/youtube-schedule-video
  

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[github-actions]

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[taipeicoder]

Works as intended.