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?
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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]

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.

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

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[github-actions]

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[lhkowalski]

Am I dreaming? 💯

[lhkowalski]

Sub-feature control. Nice!

[lhkowalski]

Code looks good and is working as expected.

The API response looks great, very well done on adding the default values for older features, as well as using nested features.

When the global "Write Brief" feature is disabled, the section on the UI is gone, as expected:

API Response	UI

When some sub-feature of "Write Brief" is disabled, the respective options are gone on the UI, as expected:

API Response	UI

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An aside: after disabling some sub-features on the backend, I went to the editor and could see them visible for half a second, while the feature data was loaded. This is expected, as you discussed earlier today, and not a problem. We can make it better with loading states in the future, for example.