Function create_payment_endpoint 

pub async fn create_payment_endpoint(
    session: &PubkySession,
    app_id: &PaykitAppId,
    identifier: PaymentEndpointIdentifier,
    payload: PaymentEndpointPayload,
) -> Result<()>

Creates a public Payment Endpoint only when it is currently absent.

Session creation, capability scope, and key rotation remain the caller’s responsibility.