public void ToDigitsOnly_WithNonDigits_ReturnsNormalized(string cardNumber, string expected)
{
var result = CreditCardNumberFormatter.ToDigitsOnly(cardNumber);
Assert.Equal(expected, result);
}
public void ToDigitsOnly_WhenValid_ReturnsUnchanged(string cardNumber)
{
var result = CreditCardNumberFormatter.ToDigitsOnly(cardNumber);
Assert.Equal(cardNumber, result);
}