Protected packages: Use CheckRuleExistenceService in npm push protection

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What does this MR do and why?

• Integrate CheckRuleExistenceService that was introduced in !135475 (merged)
• This MR needs to wait for Protected packages: PyPI package push protection (!135475 - merged)

🛠 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

No frontend change. Only a small backend refactoring

How to set up and validate locally

1. Execute the test for Npm::CreatePackageService

bundle exec rspec spec/services/packages/npm/create_package_service_spec.rb

Related to #323970