Sets the payment method for a session. Transitions the session from requires_payment_method to waiting_payment and returns payment data for the selected rail. EVM and Tron return receiver addresses, Solana returns a serialized transaction to sign, and exchanges return a hosted payment URL plus waiting copy.
32-character hex session ID
"a1b2c3d4e5f6a1b2c3d4e5f6a1b2c3d4"