Refactor service layer: decouple domain logic to lib

What does this MR do and why?

• Refactor service layer: decouple domain logic to lib
• Clean up tests - remove duplication of testing between service and domain (lib) layers
• Improve RSpec patterns - DRY up duplicate test logic via context and shared_example.
• Ensure 100% code coverage via unit tests at each layer

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.