Bug Description

ProxyAgent, EnvHttpProxyAgent, RetryAgent can't be passed to new MockAgent({ agent }), because they're missing closed and destroyed props.

Environment

Win 10, Node v20.13.1

Additional context

Not sure if I am approching this correctly, but I would like to handle non-itercepted requests exactly the same way as they would be by the app. So my assumption is that I am supposed to pass an agent used by my application to MockAgent and it will be used for non-intercepted requests.