fix offline user connection, add tests

Signed-off-by: Sahil Kumar <[email protected]>

packages/stream_chat/lib/src/core/error/stream_chat_error.dart

@@ -45,6 +45,9 @@ class StreamWebSocketError extends StreamChatError {
   /// Response body. please refer to [ErrorResponse].
   final ErrorResponse? data;
 
+  ///
+  bool get isRetriable => data == null;
+
   @override
   List<Object?> get props => [...super.props, code];