Subscriptions: excerpt not displayed in Email preview when excerpt is set in options

### Impacted plugin

Jetpack

### Quick summary

Jetpack's Newsletter feature allows you to only set an excerpt of your posts in the subscription emails. This is an option you can set in Newsletter settings:

That option works well for the emails that get sent out after you hit Publish, but doesn't work in the email previews (both in the block editor and the test email you can send).

### Steps to reproduce

1. Go to Jetpack > Newsletter
2. Enable Newsletters
3. Scroll down the page and switch the settings to only send out excerpts
4. Go to Posts > Add New
5. Create a new post, with some content and a custom excerpt
6. Check the email preview

=> You'll see that the preview shows the full content instead of the excerpt. 🐛

7. Now close the preview, and hit publish
8. The Pre-publish panel will appear
9. Scroll down to the Newsletter section
10. Click to send a test email

=> The email you'll receive will not use the custom excerpt 🐛

11. Now hit Publish and publish the post for good.
12. Check the email you've received

=> It uses the custom excerpt as expected. ✅

### Site owner impact

Fewer than 20% of the total website/platform users

### Severity

Minor

### What other impact(s) does this issue have?

No revenue impact

### If a workaround is available, please outline it here.

_No response_

### Platform (Simple and/or Atomic)

Simple, Atomic, Self-hosted

Reproduce the issue through the Jetpack > Newsletter settings and both email preview paths described in the steps. Compare how the preview and test email obtain post content with the published email; done means both use the custom excerpt when excerpt-only sending is enabled.

Written by the indexing model from the issue text.