AI Assistant: Add FeatureControl to Write Brief

[dhasilva]

Fixes #39167

Proposed changes:

• Uses the AI Helper to get feature availability from the backend
• Checks those values before enabling Write Brief or any individual highlight type

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:

• Apply this PR to your sandbox (testing locally alone won't work, as the AI helper class needs to be in sync with the backend)
• Go to the block editor
• Check that Breve is enabled and all its features (spelling is still under beta flag, check will be removed in a follow-up)
• Edit the value of write-brief or any of its subfeatures in D160005-code
• Check that the features are enabled or disabled according to the function

Only Write Brief checks for the function return value, other features will work regardless of their value.

[github-actions[bot]]

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-get-features-control branch.

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

  bin/jetpack-downloader test jetpack update/jetpack-ai-get-features-control
  

Interested in more tips and information?

• In your local development environment, use the jetpack rsync command to sync your changes to a WoA dev blog.
• Read more about our development workflow here: PCYsg-eg0-p2
• Figure out when your changes will be shipped to customers here: PCYsg-eg5-p2

[github-actions[bot]]

Thank you for your PR!

When contributing to Jetpack, we have a few suggestions that can help us test and review your patch:

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This comment will be updated as you work on your PR and make changes. If you think that some of those checks are not needed for your PR, please explain why you think so. Thanks for cooperation :robot:

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