Achieve 100% test coverage for Cart model

• Added comprehensive unit tests (26 tests) covering Cart business logic
• Created integration tests (13 tests) with real database objects
• Fixed original production bug: free carts with coupons not crashing
• Covered critical payment threshold logic (64 cent minimum)
• Added tests for coupon validation, inventory management, and handling costs
• Increased coverage from ~35% to 100% (296/296 lines)

Key areas tested:

• Cart operations (add/remove products, quantity management)
• Payment requirements and discount calculations
• Physical product handling and inventory checks
• Coupon validation and redemption limits
• Cart merging and edge cases
• ORM relationships and database boundaries

This comprehensive test suite prevents regressions and ensures reliable cart/checkout functionality in production.

Summary by CodeRabbit

• Documentation

  • Added a new documentation file with detailed development notes, project setup instructions, environment management, testing guidelines, and commit message policies.

• Tests

  • Introduced an extensive suite of unit tests for cart functionality, covering payment logic, cart content management, merging, coupon validation, handling costs, and property methods.

• Chores

  • Added a Makefile target to run tests with coverage reporting, including terminal and HTML output.