Post by Email: oEmbed links get hyperlinked instead of embedded if included in emailed posts

When including a link in the email that would automatically embed via oEmbed if pasted in the editor, the link gets posted as a hyperlink instead, preventing it from being embedded.

#### Steps to reproduce the issue

1. Go to https://wordpress.com/settings/writing/{site_URL} and enable post by email. Copy the secret email address.
2. Create a new email in your email client. Paste in the secret email address. Add a subject line (post title) and some content, including a Youtube link on its own line. (also confirmed with other oEmbed-supported links for Twitter, Instagram, and Spotify)
3. Send the email
4. Wait for the post to publish, and then view the published post.

#### What I expected
To see the content I emailed, including the embedded Youtube video

#### What happened instead
I see the content, but the Youtube link appears as a clickable hyperlink, and is also hyperlinked when I open the post in the editor. Only after I un-link it in the editor does the content correctly embed.

Reported by a WordPress.com user in https://en.forums.wordpress.com/topic/embed-youtube-video-via-post-by-email/, but I was able to replicate on both WordPress.com and a Jetpack-connected site.

No source file or test is named in the issue. Start by tracing Jetpack’s post-by-email handling for emailed oEmbed URLs, then reproduce the issue with a YouTube link and compare the published post with editor behavior; done means supported oEmbed links are embedded without manual unlinking.

Written by the indexing model from the issue text.