Add image refinement flow to our image generation screens

[dkotter]

Important

Image editing only works with the Google Provider and requires some changes to be released there first. You can manually copy those changes over to test (which is what I've done) but easiest to wait for those changes to be released.

What?

Closes #279

Partially closes #270

Add a "Refine" flow to our existing image generation screens

Why?

It's likely that an image you generate doesn't match exactly what you want the first time. While you can edit the prompt and try again, sometimes the image generated is pretty close and needs just a few refinements. This flow is now supported when generating images, both in the editor and directly in the Media Library.

How?

• Update our generate-image Ability to support passing in a reference base-64 encoded image. If that is passed, we add that as a file to our prompt
• In our image generation UI, add a "refine" state that allows you to enter a refinement prompt and send that along with the generated image (this was previously added in Inline image generation #235 but removed)
• When a refinement comes back, show the original image next to the refined image for easy comparison
• When importing an image, keep track of each prompt used so we can store that with the image. Previously we stored the original prompt only, now we store that plus any refinement prompts

Use of AI Tools

Pulled my original code from #235 for most of this. Used GPT-5.3 Codex in Cursor to refine and fix a few things with manual review, testing and cleanup done by me

Testing Instructions

1. Install the AI Provider for Google plugin
2. Pull this PR down and run npm install && npm run build
3. Add valid Google credentials
4. Turn on the Image Generation experiment
5. Add an image block into a post and click on the Generate Image button
6. Add a prompt to generate an image
7. Click the Refine Image button and add another prompt
8. Choose to keep that image, refine again or generate again (which will send the original image along with the refinement prompt, basically a "try again" approach)
9. Ensure all those buttons work as expected and the image imports with proper history
10. Do the same thing with the stand-alone image generation screen at Media > Generate Image

Screenshots

[codecov]

Codecov Report

❌ Patch coverage is 82.75862% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 58.13%. Comparing base (74dcf0e) to head (ab60a51).

Files with missing lines	Patch %	Lines
includes/Abilities/Image/Generate_Image.php	82.75%	5 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##             develop     #292      +/-   ##
=============================================
+ Coverage      57.72%   58.13%   +0.40%     
- Complexity       567      572       +5     
=============================================
  Files             36       36              
  Lines           2933     2952      +19     
=============================================
+ Hits            1693     1716      +23     
+ Misses          1240     1236       -4     

Flag	Coverage Δ
unit	58.13% <82.75%> (+0.40%)	⬆️

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