Generate a pytest suite that tests behavior, not implementation

Write a pytest test suite for the code below. Test the documented behavior and contract, not the internal implementation — if the function's logic were rewritten to do the same thing differently, these tests should still mostly pass.

CODE UNDER TEST
def parse_discount_code(code: str, cart_total: float) -> DiscountResult: ...

EXTERNAL DEPENDENCIES TO ISOLATE
a database session, an external tax-rate API call. Mock or fake these — do not let the test suite depend on a real network call, real file I/O, or a real database. Use pytest-mock's mocker fixture or unittest.mock.patch.

REQUIREMENTS
1. Use @pytest.mark.parametrize for any case that shares the same assertion shape with different inputs — don't hand-write five near-identical test functions.
2. Use fixtures for setup shared across tests, and scope each fixture (function, module, session) to the narrowest scope that's still correct.
3. Cover: the happy path, expired discount codes, codes with leading/trailing whitespace, a valid code where cart_total is below the minimum spend, and at least one case not listed that you judge worth testing — name why you added it.
4. For anything defined with async def, use pytest-asyncio's async test support — don't wrap async code in asyncio.run() inside a synchronous test.
5. Name each test after the behavior it verifies (e.g. test_returns_empty_list_when_no_matches), not test_1 or test_case_a.
6. Assert on outcomes and raised exceptions with pytest.raises, never on internal call counts unless the actual requirement is "this must call X exactly once."

OUTPUT FORMAT
The test file, followed by a short list of anything you could not test without more context (e.g. a fixture needing real credentials) and why.