Function probe_response_json 

fn probe_response_json(
    instance_id: &str,
    hostname: &str,
    version: &str,
    port: u16,
    auth_key: &str,
) -> Value

Build the JSON response payload for a valid probe. Pure/free function (no &self) so it is trivially testable without spinning up a full LanDiscovery (which owns a real mDNS ServiceDaemon). LanDiscovery’s production loop calls this via build_probe_response.