Function update_payment_endpoint 

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

Replaces a public Payment Endpoint only at the supplied strong revision.

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