Map auth proxy setup failures to specific support error_ids

[faisalahammad]

Summary

Addresses issue:

• Separate or utilize different message IDs for "The request to the authentication proxy has failed with an error" related occurrences #13133

Relevant technical choices

Setup "Get help" links for auth-proxy failures previously used the raw WP_Error code, or remapped only request_failed to the parent guide id. Transport failures share generic codes (http_request_failed, http_request_not_executed), so users landed on the wrong or missing support guide.

Setup now resolves error_id from the full WP_Error (message patterns first, then known codes, then the existing request_failed parent remap / pass-through). Doc anchor ids from the issue are used so links target the matching sub-guides. Custom proxy codes from #11752 still pass through unchanged. User-facing error text is unchanged.

PR Author Checklist

• My code is tested and passes existing unit tests.
• My code has an appropriate set of unit tests which all pass.
• My code is backward-compatible with WordPress 5.2 and PHP 7.4.
• My code follows the WordPress coding standards.
• My code has proper inline documentation.
• I have added a QA Brief on the issue linked above.
• I have signed the Contributor License Agreement (see https://cla.developers.google.com/).

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Code Reviewer Checklist

• Run the code.
• Ensure the acceptance criteria are satisfied.
• Reassess the implementation with the IB.
• Ensure no unrelated changes are included.
• Ensure CI checks pass.
• Check Storybook where applicable.
• Ensure there is a QA Brief.
• Ensure there are no unexpected significant changes to file sizes.

Merge Reviewer Checklist

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• Ensure the corresponding issue has a ZenHub release assigned.
• Add a changelog message to the issue.

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