Subscriptions - YouTube embed not showing with Classic Editor

#### Steps to reproduce the issue

1. Use Classic block
2. Paste YouTube link
3. Publish
4. Check post in email

#### What I expected

To see the YouTube video embed in the email

#### What happened instead

No YouTube embeds, just blank space.

When checking the Classic block in HTML, here are the embeds format that won't work:

```

https://youtu.be/Bf_hHU1Yu7o

[embed]https://youtu.be/WzCL3FM9dDc[/embed]

But using the YouTube shortcode works ok:

`

[youtube https://youtu.be/Bf_hHU1Yu7o]

#### Workarounds

* Use the block editor
* If still using the Classic block, then use the YouTube shortcode

Ticket: 3602966-zd-woothemes

No source file or test is named. Start by reproducing the issue with a Classic block containing both YouTube URL formats, publish it, and inspect the resulting email alongside the working YouTube shortcode and block-editor cases. Done means supported YouTube embeds render in the email instead of leaving blank space.

Written by the indexing model from the issue text.