Fix admin label fallback for regioned locales

[Sogl]

Summary

• Expand the admin label language fallback chain so regioned admin preferences also try the primary subtag before English.
• Keep the existing primary-to-region expansion for Admin Next dictionaries such as en -> en-US.
• Add unit coverage for ru-RU, es-MX, and bare en fallback chains.

This lets dual-target plugins keep normal short-code language files such as ru.yaml / en.yaml unless they intentionally need region-specific translations.

Testing

• /usr/local/opt/php@8.3/bin/php -l classes/Api/Controllers/TranslatesAdminLabels.php
• /usr/local/opt/php@8.3/bin/php -l tests/Unit/Controllers/TranslatesAdminLabelsTest.php
• /usr/local/opt/php@8.3/bin/php vendor/phpunit/phpunit/phpunit --configuration phpunit.xml tests/Unit/Controllers/TranslatesAdminLabelsTest.php
• GRAV_ROOT=/Users/artyom/domains/palagin2 /usr/local/opt/php@8.3/bin/php vendor/phpunit/phpunit/phpunit --configuration phpunit.xml tests/Unit/Controllers/TranslatesAdminLabelsTest.php

Full suite check:

• GRAV_ROOT=/Users/artyom/domains/palagin2 /usr/local/opt/php@8.3/bin/php vendor/phpunit/phpunit/phpunit --configuration phpunit.xml currently fails on the unchanged develop baseline with 8 existing auth errors in ApiKeyManager and 1 existing CORS exposed-headers failure. The same failures reproduce after stashing this PR's changes.

[rhukster]

Thanks for this, and for the thorough tests. I reproduced it on a clean develop: a regioned admin preference like ru-RU built the chain ['ru-RU', 'en', 'en-US'] and skipped the bare ru, so a plugin shipping a plain ru.yaml never got a look in and labels dropped to English. Your two new cases fail before the change and pass after it, exactly as expected.

Merged, and I added a CHANGELOG entry for the next release. Appreciate the clear write-up and the fix keeping the existing en -> en-US expansion intact.