ConstReadonlyPAYMENT_REQUIRED: "PAYMENT-REQUIRED"Header for 402 response containing payment requirements
ReadonlyPAYMENT_RESPONSE: "PAYMENT-RESPONSE"Header for settlement response after successful payment
ReadonlyPAYMENT_SIGNATURE: "PAYMENT-SIGNATURE"Header for client payment signature/payload
HTTP header names for x402 protocol Spec compliance: x402 v2 - HTTP Transport