AI Assistant: Add chat history to inline extension

[dhasilva]

Fixes #37410
Fixes #37428
Depends on D149047-code

Proposed changes:

• Adds a chat history, limited to the last 20 consecutive requests

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:

To be tested with D149047-code

• Apply the diff above and sandbox the public API
• Enable debug logging
• On a Heading, send some consecutive requests
• Check the debug messages, see that each request adds the previous response and a new message on the array
• Check that the responses make sense
• Check that the content can be edited between requests and the current content is sent.

An example of history that was not possible before:
Translate to: Portuguese.
Scratch that, go back to how it was before, but in all caps

[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-inline-extension-heading-history branch.

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

  bin/jetpack-downloader test jetpack update/jetpack-ai-inline-extension-heading-history
  

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[github-actions]

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[lhkowalski]

I tested the code and did more than 10 interactions to check how the 20-messages cap was going to work.

I confirmed that:

• each request adds the previous response and a new message on the array
• the responses make sense
• the content of the heading can be edited and the new version is considered by the Assistant

The history works in a nice way. I tested multiple translations, tone changes, adding and removing emojis, than asked to revert to the original language (it worked), asked to add back the first emoji we tried (it worked), even when I updated the heading myself.

Really nice work!