This endpoint validates credit cards and returns a response of either 'Valid' or 'Invalid.' It's intended for those who want to save cards for recurring payments. By validating the card, merchants can ensure that they only save valid cards. Additionally, the merchant can save the card in a single step by passing 'save_card = true'. There are three possible responses: INITIATED: Tap provides a payment URL (transaction.url) to initiate verification. The customer should be redirected to this URL to complete the verification; VALID: The card is valid; INVALID: The card is invalid.