Exercise 2: An Upper-Bound Contract Check — Possible Solution
====================================================================

THE EXTENDED CONTRACT TEST
------------------------------
    def contract_test_discount_strategy(strategy):
        result = strategy.apply(DISCOUNT_CONTRACT_INPUT)
        errors = []
        if not isinstance(result, (int, float)):
            errors.append(f"apply() must return a number, got {type(result).__name__}")
        else:
            if result < 0:
                errors.append("apply() must not return a negative total")
            if result > DISCOUNT_CONTRACT_INPUT:
                errors.append(f"apply() must not return a total GREATER than the input, "
                               f"got {result} > {DISCOUNT_CONTRACT_INPUT}")
        return errors

    class InflatingDiscount(DiscountStrategy):  # deliberately broken
        def apply(self, total): return total * 1.1  # INCREASES the price

RESULTS
------------------------------
    NoDiscount: PASS
    GoldDiscount: PASS
    PlatinumDiscount: PASS
    InflatingDiscount: FAIL: ['apply() must not return a total GREATER than the
    input, got 110.00000000000001 > 100.0']

All three existing, legitimate strategies still pass the extended
contract - none of them was ever close to violating the new upper-
bound rule. The deliberately broken InflatingDiscount is caught
immediately.

WHY THIS WORKS AS AN ANSWER
------------------------------
This demonstrates a contract can be extended over time as new
assumptions about "correct" become explicit, the same way Chapter 5's
own contract test evolved when a new required field was added. It also
reinforces Step 3's own real value: InflatingDiscount is exactly the
kind of bug (a genuinely wrong new DiscountStrategy implementation)
that a contract test catches in isolation, before it's ever wired into
a real checkout flow where the consequence would be a customer charged
more than the subtotal they started with.