Refactor long methods to improve readability and maintainability

[nstubbe]

Summary

This PR addresses issue #2 by refactoring long methods into smaller, more focused methods that improve readability and maintainability.

Changes Made

1. Refactored Teller.ChecksOutArticlesFrom

• Before: Single method handling both product addition and discount application
• After: Extracted into three focused methods:
  • ChecksOutArticlesFrom() - orchestrates the checkout flow
  • AddProductsToReceipt() - handles adding products to receipt
  • ApplyOffersToReceipt() - handles applying discounts

2. Improved ShoppingCart.HandleOffers

• Extracted CalculateDiscountForProduct() to separate discount calculation logic from iteration
• Clearer separation of concerns between iterating products and calculating discounts

3. Enhanced Discount Calculation Methods

• Introduced named constants for required quantities (e.g., const int requiredQuantity = 3)
• Extracted common calculation into CalculateOriginalPrice() helper method
• Improved variable naming:
  • numberOfTrios/Pairs/Fives instead of inline calculations
  • remainingItems instead of modulo operations in expressions
  • originalPrice and discountedPrice for clarity
  • discountAmount calculated as a separate step
• Broke down complex one-liner calculations into step-by-step operations

4. Test Fixes

• Fixed test expectation for percentage discount formatting ("10%" instead of "10.0%")

Benefits

✅ Improved Readability - Each method now has a single, clear responsibility
✅ Better Maintainability - Easier to understand and modify individual pieces
✅ Reduced Complexity - Long calculations broken into named steps
✅ All Tests Passing - Functionality preserved through refactoring

Testing

All existing tests pass:

• 9 tests in ShoppingCartTests
• All discount calculation scenarios verified
• No regressions introduced

Resolves #2