JSON API: preserve valid sub-400 WP_Error statuses in serializable_error()

[darssen]

Proposed changes

PR #50077 hardened WPCOM_JSON_API::serializable_error() so that a WP_Error can no longer serialize a bogus status (the incident where an array without a status_code key cast to 1). That change did two things: it made the status extraction robust, and it also coerced every status below 400 up to 400.

The second part was too broad. Some callers return a WP_Error with a status below 400, for example a 302 redirect from the checkout flow, and a couple of older endpoints that return 100 or 200 with an error. Coercing those to 400 changed their behavior and started failing the Payments PayPal Express test suite.

This PR keeps the robust extraction from #50077 and narrows the default back to only handle a missing or invalid status. A value that is missing, non-numeric, or otherwise resolves to 0 still falls back to 400 (so the original incident stays fixed), but a valid HTTP status code, including one below 400, is now left as the caller set it.

Related product discussion/links

• Regression reported in the payments-dev thread: p1782993711546339-slack-C096PD42U
• Original change: JSON API: serializable_error() valid-int-only status hardening #50077

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

No.

Testing instructions

• Run the unit tests:

  jetpack docker phpunit jetpack -- --filter=WPCOM_JSON_API_Serializable_Error_Test
  

  14 tests pass. They cover the incident case (an array without status_code, a non-numeric value, and no data all default to 400), and that valid statuses, including 100, 200, and 302, pass through unchanged.

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[martian77]

Although I agree with the spirit of the original fix, the WP_Error code supports the other codes and has been used this way in WPCOM code so this is probably the correct way to guard against invalid responses.

Thanks for making this change! 🙇

[jp-launch-control]

Code Coverage Summary

This PR did not change code coverage!

That could be good or bad, depending on the situation. Everything covered before, and still is? Great! Nothing was covered before? Not so great. 🤷

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