AI Assistant: Load Breve spelling dictionaries from CDN

[dhasilva]

Fixes #38887

Proposed changes:

• Loads the dictionary JSON from the backend, saving it to localStorage and reusing it on page refreshes

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:

• Without sandboxing the public API, go to the block editor, with the Spelling Mistakes feature turned on
• Check that the dictionary fails to load, but nothing breaks
• Apply D158668-code and sandbox the public API
• Reload the block editor
• Check that the dictionary is loaded, with only one request to the endpoint. Note: The highlights are not properly re-loaded, but they appear if you click on a block. This will be tackled later.
• Reload the page
• Check that the dictionary is loaded from localStorage, without any requests
• Check that the typos are highlighted

[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-breve-load-dictionary-from-server branch.

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

  bin/jetpack-downloader test jetpack update/jetpack-ai-breve-load-dictionary-from-server
  

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[github-actions]

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[CGastrell]

Code looks good and works as described. Nice implementation!

[CGastrell]

LGTM! Load works nicely