Refactor: Split monetizable_spec.rb into smaller files

[RubenIgnacio]

The monetizable_spec.rb file had grown to over 1,000 lines, making it increasingly difficult to navigate, maintain, and add new test cases. To improve readability and maintainability, I refactored the test suite by splitting monetizable_spec.rb into smaller, focused files based on the functionality they test.

This is purely a structural refactor. No test logic was changed, added, or removed. I extracted specific describe and context blocks from the main spec file into a new monetizable/ directory. The specs are now organized functionally:

monetizable
├── assignment_spec.rb
├── currency_for_spec.rb
├── error_handling_spec.rb
├── monetized_attributes_spec.rb
├── read_monetized_spec.rb
├── register_currency_spec.rb
├── validations_spec.rb
└── write_monetized_spec.rb
monetizable_spec.rb