Clean up test extraction folders in tear_down

[erseco]

First of a stack of eight small PRs from a modernization/security pass. Each PR targets the previous branch; merge them in order.

Problem

ElpUploadHandlerTest::test_delete_extracted_folder_nonexistent_dir failed intermittently on full-suite runs and passed when run alone.

Several tests create uploads/exelearning/{hash}/ fixtures with fixed hashes (ffff…, aaaa…, cccc…, 7777…). They remove the fixtures inline, after the assertions, or not at all. The wp-env uploads directory persists across runs, so an aborted or failing run left, for example, ffff…f/ behind (from RestApiTest::test_save_elp_file_cleanup_with_old_extraction). That broke the precondition of the handler test on the next run.

Change

• ElpUploadHandlerTest, RestApiTest and ShortcodesTest register every fixture path and delete it in tear_down(). This is the pattern StaleContentRedirectTest already uses, with ExeLearning_Styles_Service::recursive_delete().
• The "missing folder" test uses a unique hash, so no other test can collide with it.

Tests only; no runtime change.

Verification

• ffff…f/index.html was seeded by hand in the tests container. The full suite then ran green twice in a row: OK (943 tests, 2074 assertions).
• No uploads/exelearning/ leftovers after the runs.

Stack

1. Clean up test extraction folders in tear_down #136 Clean up test extraction folders in tear_down
2. Apply the style package allowlist to nested entries #137 Apply the style package allowlist to nested entries (security)
3. Check who sends postMessage to the editor modal and bridge #138 Check who sends postMessage to the editor modal and bridge (security)
4. Harden the release workflows against injected values #139 Harden the release workflows against injected values
5. Keep make up working with a dirty editor clone or no network #140 Keep make up working with a dirty editor clone or no network
6. Share one recursive delete and validate hashes before deleting #141 Share one recursive delete and validate hashes before deleting
7. Remove unreachable scaffolding and clean options on uninstall #142 Remove unreachable scaffolding and clean options on uninstall
8. Make AGENTS.md the single agent guide and import it from CLAUDE.md #143 Bring CLAUDE.md and AGENTS.md in line with the code

[github-actions]

Test in WordPress Playground

Test the plugin with the code from this branch:

ℹ️ The eXeLearning editor is fetched from the shared release and unpacked into the plugin when the playground boots, so the first load may take a few extra seconds. ELP upload, shortcode, Gutenberg block and preview work normally.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.87%. Comparing base (3410936) to head (704e330).

Additional details and impacted files

@@            Coverage Diff            @@
##               main     #136   +/-   ##
=========================================
  Coverage     96.87%   96.87%           
  Complexity      864      864           
=========================================
  Files            39       39           
  Lines          4323     4323           
=========================================
  Hits           4188     4188           
  Misses          135      135           

Flag	Coverage Δ
javascript	95.70% <ø> (ø)
php	97.25% <ø> (ø)

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