fix: defer protected content status check until init

[rickalee]

Summary

• move the Protected Content active-state check out of the Co-Authors Plus constructor
• evaluate the dependency during feature requirements validation on init
• add regression coverage and an unreleased changelog entry

This keeps feature registration available before ElasticPress processes registered features while avoiding ElasticPress translation calls during plugins_loaded. The dependency state is still populated before the Co-Authors Plus feature setup runs.

Closes #210

Testing instructions

1. Install Composer dependencies.
2. Start a supported Elasticsearch instance and set EP_HOST.
3. Run vendor/bin/phpunit tests/phpunit/feature/TestCoAuthorsPlus.php.
4. Run vendor/bin/phpcs includes/classes/Feature/CoAuthorsPlus.php tests/phpunit/feature/TestCoAuthorsPlus.php --runtime-set testVersion 7.4-.

Test results

• Co-Authors Plus suite: 9 tests, 25 assertions passed.
• PHPCS: passed for both changed PHP files.
• Full PHPUnit suite: 185 tests completed with 5 unrelated local-environment failures in existing GeoLocation and User analyzer tests using Elasticsearch 8.15 and PHP 8.5.