Replace Assert::assertEquals with Assert::assertEqualsWithDelta (epsilon 0.0001)
in assertSubTotal, assertTotal, and assertTotalQuantity to avoid IEEE 754
floating-point equality pitfalls. Add @internal docblock to fake() to signal
it is test-only. Add regression test for consecutive withItems calls.
Adds Cart::fake() which swaps the active driver to NullDriver and returns
a CartFactory fluent test builder. Adds assertion methods (assertContains,
assertCount, assertTotalQuantity, assertSubTotal, assertTotal,
assertConditionApplied, assertEmpty, assertNotEmpty) to Cart for use in
tests. CartFactory provides withItems(), withCondition(), and withCoupon()
as chainable helpers for seeding cart state.
Joel Male