Fix GOOWOO-4: allow scripts and styles to share an asset handle

[gabriel-fuentes]

Changes proposed in this Pull Request:

Closes #129.

AssetsHandler stored assets in a flat handle -> Asset map and threw InvalidAsset on any duplicate handle. WordPress core keeps separate registries for scripts and styles so same-handle pairs are normal there. The original report describes that case, and Zendesk #11272914 confirms a real fatal in production when GfW is active alongside Enhanced Media Library (~70K installs): two code paths register the same handle within a single request, the second one throws, and the customer hits a white screen on wp-admin/options-general.php?page=media.

Change AssetsHandler::$assets to array<string, Asset[]> so a handle can hold multiple assets of different classes. Same handle + same class is now a silent no-op (idempotent), matching WordPress's own wp_register_script / wp_register_style behavior. AssetsHandlerInterface is unchanged, so existing callers see no signature change.

Screenshots:

n/a (no UI change).

Detailed test instructions:

PHPUnit (covers the AC + idempotency):

1. composer install
2. vendor/bin/phpunit --bootstrap vendor/autoload.php --no-configuration tests/Unit/Assets/AssetsHandlerTest.php
3. Expect 9 passing tests, 19 assertions.

Manual repro of Zendesk #11272914:

1. WP 6.7+, WC 10.7+, GfW (this branch), Enhanced Media Library 2.9.4 all active.
2. Visit wp-admin/options-general.php?page=media as an admin.
3. The page loads without a fatal. Without this fix, the same setup throws InvalidAsset: The asset handle "google-listings-and-ads" already exists.

The AC scenario (registering a ScriptAsset and a StyleAsset under the same handle) is covered by test_allows_script_and_style_with_same_handle and friends.

Additional details:

The fix also lets future GfW code register a StyleAsset under the same handle as its matching script, which is what the issue's original AC asks for. The current google-listings-and-ads script + google-listings-and-ads-css style split was a workaround for this limitation.

Changelog entry

Fix - Allow scripts and styles to share an asset handle, preventing fatal errors when multiple components register an asset under the same handle.

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Created during the Woo Bug Blitz with Claude Code via team Ceres' /bug-blitz skill.

Drafted with AI; reviewed by me.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 67.5%. Comparing base (5f2c6bf) to head (a50ba86).

Additional details and impacted files

@@             Coverage Diff             @@
##             develop   #3477     +/-   ##
===========================================
+ Coverage       66.7%   67.5%   +0.9%     
- Complexity      5514    5520      +6     
===========================================
  Files            923     531    -392     
  Lines          28735   22058   -6677     
  Branches        1694       0   -1694     
===========================================
- Hits           19164   14899   -4265     
+ Misses          9319    7159   -2160     
+ Partials         252       0    -252     

Flag	Coverage Δ
js-unit-tests	?
php-unit-tests	67.5% <100.0%> (+0.1%)	⬆️

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Files with missing lines	Coverage Δ
src/Assets/AssetsHandler.php	79.5% <100.0%> (+79.5%)	⬆️

... and 393 files with indirect coverage changes

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