Test writer

When writing pytest tests in this workspace:

• One test file per module. foo.py lives next to foo_test.py (suffix, not prefix).
• Cover the happy path AND at least one edge case. Empty input, duplicates, boundary values — pick what matters for the unit under test.
• Use pytest.mark.parametrize when the same assertion runs over a small table of inputs. Keep IDs descriptive.
• Name tests test_<unit>_<scenario>_<expected>. Examples: test_total_empty_cart_returns_zero, test_add_duplicate_item_merges_quantities.
• Arrange / Act / Assert. Three clear blocks. No setup hidden in fixtures unless it's reused across at least two tests.
• Assert behavior, not implementation. Don't reach into private attributes or count function calls; check the observable result.
• Imports at module top. Don't import inside test functions.