Create a demo MPP charge

DEMO-ONLY MPP (Machine Payment Protocol) charge. A bare request returns HTTP 402 with MPP payment requirements; a retry carrying a Payment header returns a canned demo charge. The payment rail is the Tempo testnet (Stripe MPP testnet) and no settlement is verified. Forter is a fraud-prevention platform and does not sell goods or take real payment. MPP discovery for this operation is the x-payment-info extension below.