Search: Fix search on private WordPress.com-on-Atomic sites

[jsnmoon]

Fixes #24197.

Changes proposed in this Pull Request:

Fixes search on private WordPress.com-on-Atomic (WoA) sites by signing the requests with a blog token instead of a user token.

Also changes the local API URL to the one provided by the search package instead of the one provided by the Jetpack plugin.

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

No.

Testing instructions:

Note: This PR is only testable by a12s.

1. Spin up a WoA site (either via Pro plan purchase + Jetpack Beta plugin install or via WoA developer blogs — p9o2xV-1r2).
2. Purchase Jetpack Search for the WoA site.
3. Navigate to /?s=. Ensure that the search interface works as expected.
4. Navigate to https://wordpress.com/settings/general/ and change site visibility to Private.
5. Repeat the site search from step 3; ensure that the search API request fails in the network console.
6. Install the Jetpack Beta plugin (if you haven't in step 1) and change the plugin branch to this one.
7. Repeat step 5; ensure that the search API request succeeds.
8. Revert the site privacy back to Public and repeat step 5, just in case.

[github-actions]

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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• ✅ Add testing instructions.
  
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[jsnmoon]

I believe this regression was introduced in #21584. @kangzj, do we know why we changed to signing with user tokens here?

[kangzj]

@jsnmoon Yep I think so, this is a bug - I guess I didn't anticipate a local user isn't necessary a WPCOM user in the previous PR.

[kangzj]

Hi @jsnmoon, could you please share more details about how to create a WoA site? I'm not sure how I set up one - should I use the sandbox, a ephemeral site or sth else?

[jsnmoon]

Hi @jsnmoon, could you please share more details about how to create a WoA site? I'm not sure how I set up one - should I use the sandbox, a ephemeral site or sth else?

The consumer-centric way would be to spin up a regular WordPress.com site, purchase a Pro plan, and install a plugin on it. I'll share the more dev-friendly method (internal to a8c) privately.

[kangzj]

Thanks for fixing this. It works for me 👍