Fix getting entityId when using mapped route attributes

[jacekkow]

When using mapped attributes (i.e. entityId becoming order.id like in the example below) entity instance is not properly set inside AdminContext.

        #[AdminRoute('/{entityId:order.id}/invoice')]
        public function renderInvoice(Order $order, AdminContext $context): Response {
            $context->getEntity()->getInstance(); // is null!
        }

This is because entityId is no longer a route attribute, order.id is.

Fix this by trying to fetch original entityId value from _route_params.

Related to #7488