Exercise 1: A PlatinumDiscount + ExpressShipping Unit Test — Possible Solution
====================================================================

THE TEST
------------------------------
    def test_platinum_discount_with_express_shipping():
        # Arrange
        items = [{'name': 'widget', 'price': 20, 'qty': 2}]
        # Act
        total = calculate_order_total(items, PlatinumDiscount(), ExpressShipping())
        # Assert
        assert total == 47.0  # 40*0.8=32, +15=47

RESULT
------------------------------
    test_platinum_discount_with_express_shipping: PASS

WHY THIS WORKS AS AN ANSWER
------------------------------
This follows the exact AAA structure and per-test isolation
established in Step 1: a fresh items list created inside the test
itself, no shared state with any other test, and the three sections
kept visually distinct. It also confirms the pricing core composes
correctly for a combination (Platinum + Express) that wasn't
specifically exercised by either of Step 1's own two original tests -
extending coverage to a new discount/shipping pairing without
introducing any new pattern beyond what Chapter 3 already established.