ping method
PINGs the server.
Returns 'PONG' if no message is provided, otherwise returns the message.
Throws a ValkeyServerException if an error occurs.
Implementation
@override
Future<String> ping([String? message]) async {
final command = (message == null) ? ['PING'] : ['PING', message];
// PING response can be Simple String or Bulk String
final response = await execute(command);
// Our simple parser will return "PONG" or the message.
return response as String;
}