Jetpack AI Image: add button entrypoint on the image block

[lhkowalski]

Fixes #38122.

Proposed changes:

• Add a "Generate with AI" button on the image block, gallery block and slideshow block when the beta flag is enabled (same as the media source menu)
• The button will launch the General Purpose Image Generator and execute all the actions setting the correct placement (a new value, block-placeholder-button)
• Make the tool works when the block demands multiple image selection, but keep inserting just one image; this will fix things for the gallery and slideshow blocks

Other information:

• Have you written new tests for your changes, if applicable?
• Have you checked the E2E test CI results, and verified that your changes do not break them?
• Have you tested your changes on WordPress.com, if applicable (if so, you'll see a generated comment below with a script to run)?

Jetpack product discussion

Does this pull request change what data or activity we track or use?

Testing instructions:

• Enable beta extensions, go to the block editor and add an image block
• Confirm you can see the "Generate with AI" button:

• Confirm the general purpose image tool will show up when you click the button
• Confirm you can request an image and then use it on the block by clicking "Insert image"
• Do the same for the following blocks:
  • Slideshow
  • Gallery

• Confirm the following blocks will not show the button:
  • Cover
  • Media & Text
  • Image Compare
  • Story
  • Tiled Gallery
  • these blocks are extended by the same code so they could show the other media sources the code adds, but I opted to limit the usage on them to focus on the core ones
• Confirm the button will not show up when beta extensions are disabled

[github-actions]

Are you an Automattician? Please test your changes on all WordPress.com environments to help mitigate accidental explosions.

• To test on WoA, go to the Plugins menu on a WordPress.com Simple site. Click on the "Upload" button and follow the upgrade flow to be able to upload, install, and activate the Jetpack Beta plugin. Once the plugin is active, go to Jetpack > Jetpack Beta, select your plugin, and enable the update/jetpack-ai-image-image-block-entrypoint branch.

• To test on Simple, run the following command on your sandbox:

  bin/jetpack-downloader test jetpack update/jetpack-ai-image-image-block-entrypoint
  

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• In your local development environment, use the jetpack rsync command to sync your changes to a WoA dev blog.
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• Figure out when your changes will be shipped to customers here: PCYsg-eg5-p2

[github-actions]

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[CGastrell]

LGTM, I'm freshly coming back so I'll leave Douglas to give the final tick.

With BETA enabled I could see/use the button on Image, Gallery and Slideshow. Button would not show on any of the other listed blocks.

Without BETA I could not find the button anywhere.

Generated images inserted fine. Editing the Gallery or Slideshow won't give you the AI generator, but I think that's by design (no UI place for it, only upload or media gallery)

[dhasilva]

LGTM