Protected packages: Align logic for protection rules across registries

What does this MR do and why?

• This MR intends to keep the code and implementation consistent across the container and the package registry; this means, we want to apply improvements from the implementation of the protected containers, see related MRs
• Renaming the scope method .push_protected_from? to .for_push_exists? in order to be alinged with the scope method used in app/models/container_registry/protection/rule.rb
• Adjust return value of scope method to reflect the same logic as in app/models/container_registry/protection/rule.rb

🛠 with ❤ at Siemens

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

MR Checklist (@gerardo-navarro)

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the javascript style guides
• Conforms to the database guides

Screenshots or screen recordings

Only backend changes

How to set up and validate locally

1. Execute the relevant tests

bundle exec rspec spec/models/packages/protection/rule_spec.rb spec/services/packages/npm/create_package_service_spec.rb

Related to #416382

💾 Database review

• Check if a protection rule for NPM packages exists.