Add comprehensive unit tests for supermarket receipt functionality

[Copilot]

This PR addresses the need for adequate unit test coverage before refactoring by implementing a comprehensive test suite for the supermarket receipt kata Python implementation.

What was added

25 comprehensive test cases covering all existing functionality:

• All discount types: THREE_FOR_TWO, TEN_PERCENT_DISCOUNT, TWO_FOR_AMOUNT, FIVE_FOR_AMOUNT
• Receipt printing functionality (previously untested)
• Edge cases: empty cart, zero prices, large quantities, fractional quantities
• Product unit handling (EACH vs KILO)
• Multiple discount scenarios

Coverage improvements

Before: 53% overall coverage with only 1 test
After: 99% coverage (171/173 lines covered)

Module	Before	After
receipt_printer.py	0%	100%
shopping_cart.py	41%	100%
model_objects.py	89%	100%
receipt.py	93%	100%
teller.py	100%	100%

Test quality features

• Clear, descriptive test names explaining exact scenarios
• Comprehensive assertions validating totals, discounts, and individual items
• Edge case coverage including boundary conditions
• Mixed discount scenarios with multiple products
• Complete documentation with README and automated test runner

Files added

• python/tests/test_supermarket.py - Main test suite
• python/run_tests.py - Automated test runner with coverage reporting
• python/tests/README.md - Complete test documentation

Usage

Run tests with coverage:

cd python
python run_tests.py

The test suite now provides a solid safety net for future refactoring, ensuring that any changes maintain existing functionality while enabling confident code improvements.

Note: During testing, documented a potential calculation issue in TWO_FOR_AMOUNT discount logic, but preserved current behavior as requested.

Fixes #5.

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