Publicize to Bluesky: post images are not pulled into Bluesky post

### Impacted plugin

Jetpack, Social

### Quick summary

When publicizing a post to Bluesky, an image can be attached to the post, but right now we only do so if the post includes a featured image. It would be nice if we could also pick images from the post content as well.

### Steps to reproduce

1. Start with a site that's connected to WordPress.com.
2. Go to Jetpack > Settings > Sharing
3. Connect your site to your Bluesky account
4. Go to Posts > Add New
5. Write a new post with a title, some content, and insert an image block. Add an image there.
6. Hit Publish

### A clear and concise description of what you expected to happen.

I would expect the post that gets publicized to Bluesky to display that image in the post preview.

### What actually happened

Instead, only a link is displayed.

### Impact

Most (> 50%)

### Available workarounds?

No but the platform is still usable

### If the above answer is "Yes...", outline the workaround.

_No response_

### Platform (Simple and/or Atomic)

Simple, Atomic, Self-hosted

### Logs or notes

Here is a screenshot of 3 different scenarios:

- A post with no image in the post content, but a featured image -> That featured image is picked in the post preview.
- A post with an image in the post content, and that same image as a featured image -> That featured image is picked in the post preview.
- A post with an image in the post content but no featured image -> **That is the bug we're talking about here**


Reproduce the issue through Jetpack > Settings > Sharing and Posts > Add New, using a post-content image without a featured image. Review the Jetpack Social Bluesky publicizing path and compare it with the featured-image case. Done means a content image appears in the Bluesky preview when no featured image is set.

Written by the indexing model from the issue text.