public void Given_CartItem_When_QuantityIncreased_Then_ReturnNewItemWithUpdatedQuantity()
{
// Arrange
var product = new Product("Milk", 1.0m);
var cartItem = new CartItem(product, 1);
// Act
var result = cartItem.IncreaseQuantity(5);
// Assert
Assert.NotSame(cartItem, result);
Assert.Equal(6, result.Quantity);
}