benecaster_rest_permission_admin()

Permission callback for an add-on REST route that requires a site admin. It checks manage_options and then a valid wp_rest nonce, exactly as core’s own admin endpoints do — this and RestController::permission_callback_admin() call the same implementation, so they cannot drift apart.

The shape is already right for direct use: WordPress passes the request to a permission_callback, so pass the function name as a string and nothing else.

Register your routes in your own namespace, not benecaster/v1. That namespace is core’s, and its route list is not a stable place for third-party endpoints to appear.