refactor!: cleanup Asset_Loader and add error checking

[justlevine]

What?

This PR dedupes the internal logic inside of Asset_Loader and adds both admin notices and _doing_it_wrong() notices when a bad asset is enqueued.

Additionally, tests have been backfilled to provide full coverage for the class.

Why?

• Improves DX for contributors
• Provides a guard against breaking changes to the experimental @wordpress/build as we proceed with the eventual migration.

(On a personal note, this happened to me when I was switching branches and took me too long to notice they were still in the old build dir.)

How?

• The unused $asset_data argument was removed (on ::enqueue_styles it was replaced with string[] $dependencies]. If there's really a need for us to enqueue an asset that's not in our build flow, we can reintroduce it then.
• Similarly, I kept things as ::enqueue_*() instead of renaming them to ::register_*() and then using native wp_enqueue_*() when the assets are actually required, and skipped introducing an ::enqueue_script_module() since at the moment this also feels like YANGI and can be done with forward-compat.

More specific hardening changes (using consts for reusable strings, making the class final, docblock typo) in the diff.

Use of AI Tools

• GitHub Copilot Autocomplete
• GLM-5.1 in Opencode to debug asset registration leaking into other tests and make the necessary fix to the test tearDown()

Testing Instructions

1. rm -rf build-scripts
2. visit the dashboard and confirm you see an admin notice. if you have WP_DEBUG_DISPLAY on you'll also see the _doing_it_wrong. Otherwise see it in tests/_output/debug.log.
3. Run npm run build and refesh the screen, to confirm the notice goes away.

Screenshots or screencast

Changelog Entry

Added - New feature.
Changed - Existing functionality.
Deprecated - Soon-to-be removed feature.
Removed - Feature.
Fixed - Bug fix.
Security - Vulnerability.
Development Update - Development related updates.

• Changed: Refactor Asset_Loader class and add error checking when dependencies are missing.

[github-actions]

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Co-authored-by: justlevine <justlevine@git.wordpress.org>
Co-authored-by: dkotter <dkotter@git.wordpress.org>

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[Copilot]

Pull request overview

Refactors Asset_Loader to dedupe asset-loading logic and introduce admin/_doing_it_wrong() feedback when enqueued build assets are missing/invalid.

Changes:

• Refactor Asset_Loader internals: centralize .asset.php loading, add handle prefix constant, add admin notice + _doing_it_wrong() reporting.
• Update enqueue_style() signature to accept explicit style dependencies (rather than reading deps from .asset.php).
• Add new integration tests covering valid/invalid/missing asset metadata scenarios.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 10 comments.

File	Description
includes/Asset_Loader.php	Refactors enqueue/localize logic, adds centralized asset metadata loading and admin notice/error logging.
tests/Integration/Includes/Asset_LoaderTest.php	Adds integration tests for enqueue/localize behavior and error handling around missing/invalid asset metadata.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 68.52%. Comparing base (4c9699f) to head (cab45b1).
⚠️ Report is 1 commits behind head on develop.

Additional details and impacted files

@@              Coverage Diff              @@
##             develop     #458      +/-   ##
=============================================
+ Coverage      67.72%   68.52%   +0.80%     
+ Complexity       959      958       -1     
=============================================
  Files             60       60              
  Lines           4836     4861      +25     
=============================================
+ Hits            3275     3331      +56     
+ Misses          1561     1530      -31     

Flag	Coverage Δ
unit	68.52% <100.00%> (+0.80%)	⬆️

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