Enqueued Assets audit: treat zero-size and no-store responses as errors

[Gilmoursa]

Summary

Fixes #2417.

perflab_aea_get_asset_size() had two TODOs in the code for cases that silently passed the audit when they should be flagged as errors:

1. Zero-size response body — a 200 response with an empty body means the asset is effectively broken. The audit was reporting it as OK with a size of 0 bytes.
2. Cache-Control: no-store — an asset that browsers are forbidden from caching is a performance problem and should surface in the audit. The audit was reporting these as OK as long as the status was 200.

Behaviour change

Zero-size: Returns WP_Error('zero_size') instead of 0.

Non-cacheable: Returns WP_Error('not_cacheable') when the response has a Cache-Control: no-store directive.

Cache-Control: no-cache (revalidate) is intentionally left as passing — the asset can still be served from cache after revalidation, so it is not a hard performance problem in the same way as no-store.

Both WP_Error results feed into the existing error display path in the audit table (red row, error message shown, recommended status), so no additional UI changes are needed.

Changes

• plugins/performance-lab/includes/site-health/audit-enqueued-assets/helper.php: implement the two checks, remove the TODO comments
• plugins/performance-lab/tests/includes/site-health/audit-enqueued-assets/test-audit-enqueued-assets-helper.php: update the zero-size assertion to expect WP_Error; add test_perflab_aea_get_asset_size_not_cacheable() covering no-store, no-store with extra directives, no-cache (should pass), and a normal cacheable response
• plugins/performance-lab/tests/data/class-audit-assets-mock-assets.php: pass headers from mocked responses through the pre_http_request filter so header-dependent tests work end-to-end

Test plan

• Run composer test:integration -- --filter test_perflab_aea_get_asset_size
• Run composer test:integration -- --filter test_perflab_aea_get_asset_size_not_cacheable
• Confirm an asset returning a zero-byte body shows as an error row in the Site Health blocking assets table
• Confirm an asset served with Cache-Control: no-store shows as an error row in the table
• Confirm an asset served with Cache-Control: no-cache still passes the audit

[github-actions]

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Co-authored-by: Gilmoursa <gilmoursa@git.wordpress.org>
Co-authored-by: westonruter <westonruter@git.wordpress.org>
Co-authored-by: tejas0306 <suhan2411@git.wordpress.org>

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 69.39%. Comparing base (347597f) to head (48748c9).
⚠️ Report is 5 commits behind head on trunk.

Additional details and impacted files

@@            Coverage Diff             @@
##            trunk    #2481      +/-   ##
==========================================
+ Coverage   69.33%   69.39%   +0.05%     
==========================================
  Files          90       90              
  Lines        7749     7763      +14     
==========================================
+ Hits         5373     5387      +14     
  Misses       2376     2376              

Flag	Coverage Δ
multisite	69.39% <100.00%> (+0.05%)	⬆️
single	35.66% <0.00%> (-0.07%)	⬇️

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[Copilot]

Pull request overview

This PR updates the Enqueued Assets Site Health audit so empty asset responses and Cache-Control: no-store responses are surfaced as asset errors instead of passing as successful zero-byte/cacheable assets.

Changes:

• Returns WP_Error for zero-size asset bodies and Cache-Control: no-store.
• Adds tests for zero-size and cache-control handling.
• Updates the audit asset test mock to pass response headers through.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
plugins/performance-lab/includes/site-health/audit-enqueued-assets/helper.php	Adds empty-body and no-store error handling in asset size retrieval.
plugins/performance-lab/tests/includes/site-health/audit-enqueued-assets/test-audit-enqueued-assets-helper.php	Updates and expands coverage for the new asset-size error behavior.
plugins/performance-lab/tests/data/class-audit-assets-mock-assets.php	Includes mocked headers in HTTP responses for header-dependent tests.

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