Connectors: Gate non-AI setting auto-registration

[jorgefilipecosta]

Summary

• keep connector setting auto-registration unchanged for AI providers
• only auto-register non-AI connector settings when plugin.is_active exists, is callable, and returns true
• add focused PHPUnit coverage for the AI and non-AI registration paths

Testing

• php -l src/wp-includes/connectors.php
• php -l tests/phpunit/tests/connectors/wpRegisterDefaultConnectorSettings.php
• vendor/bin/phpcs --standard=phpcs.xml.dist src/wp-includes/connectors.php tests/phpunit/tests/connectors/wpRegisterDefaultConnectorSettings.php
• vendor/bin/phpunit tests/phpunit/tests/connectors/wpRegisterDefaultConnectorSettings.php (fails locally because the configured database host mysql is unavailable)

[github-actions]

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