Protect Dashboard: Surface WordPress.com error detail on uptime failures

[kraftbj]

Fixes JETPACK-2964

Proposed changes

Follow-up to the Monitor section added in #53194 — this PR is stacked on that branch and targets it, so it should merge after #53194.

When either WordPress.com call behind GET jetpack/v4/protect-dashboard/uptime failed, the route returned a generic uptime_unavailable 502 and discarded the reason: the upstream status code and any WP_Error never reached the response, so a site reporting "uptime history is unavailable" left nothing to look at.

• The request helper now also returns an error string for a transport failure (the WP_Error code and message), instead of collapsing every failure to code 0.
• The 502's error data now carries upstreamStatus (the upstream HTTP status, 0 for a transport failure) and, for a transport failure, upstreamError. That is enough to tell a timeout from a 401 from a 500.

No UI change — the dashboard still shows the same generic message; the detail is for logs and support.

Related product discussion/links

• JETPACK-2964: https://linear.app/a8c/issue/JETPACK-2964

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

No.

Testing instructions

• This targets the add/protect-dashboard-monitor branch; review it on top of Protect Dashboard: Add the Monitor section #53194.
• Run the package tests: jetpack test php packages/protect -- --filter Monitor_Test — the provide_unusable_histories cases now assert the upstream detail on the 502 (a timed-out request, a 500, a 401, a 403, and an empty body each leave distinct data behind).
• On a connected site with Monitor on, hit GET /wp-json/jetpack/v4/protect-dashboard/uptime while WordPress.com is unreachable (e.g. short-circuit the request to a WP_Error or a 500). The 502 response body's data now includes upstreamStatus and, for a transport failure, upstreamError.

[github-actions]

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[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 WoA dev 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 (Jetpack), and enable the add/protect-uptime-error-detail-JETPACK-2964 branch.
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Code Coverage Summary

No summary data is available for parent commit 54c66c7, so cannot calculate coverage changes. 😴

If that commit is a feature branch rather than a trunk commit, this is expected. Otherwise, this should be updated once coverage for 54c66c7 is available.

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