Creates a recurring-payment mandate (UPI AutoPay or Card Standing Instruction). Two request shapes are supported depending on the flow:

• Seamless (mandates[0].isSeamless: "1"): client_id at the top level, mandate fields wrapped in a mandates array. Response returns a redirectURI for the customer to complete authentication.
• Non-seamless (isSeamless: "0"): all fields flat at the top level, no mandates wrapper. Response returns a generated mandate_link instead.