Fix rubocop issues for Npm::CreatePackageService

What does this MR do and why?

• Resolving rubocop issues to clean up the code
• Preparing code for easier refactoring step in MR !146089 (merged)
• Extracting some changes from the !146089 (merged) in a separate MR (this MR)

🛠 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. No frontend changes.

How to set up and validate locally

1. Run the fixed test. It should succeed ;-)

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

Related to #323970