fix: replace first and last name with single field (#4915)

mobile/openapi/test/sign_up_dto_test.dart

@@ -21,13 +21,8 @@ void main() {
       // TODO
     });
 
-    // String firstName
-    test('to test the property `firstName`', () async {
-      // TODO
-    });
-
-    // String lastName
-    test('to test the property `lastName`', () async {
+    // String name
+    test('to test the property `name`', () async {
       // TODO
     });