RED: Test exposing custom delimiter bug

- Test expects '//;\n1;2' to return 3
- Currently fails with FormatException
- Custom delimiter extraction is commented out
- Tries to parse '1;2' as single number, fails

test/string_calculator_test.dart

@@ -27,5 +27,10 @@ void main() {
       expect(calculator.add('1,2,3'), equals(6));
       expect(calculator.add('5,10,15,20'), equals(50));
     });
+
+    test('custom delimiter works correctly', () {
+      expect(calculator.add('//;\n1;2'), equals(3));
+      expect(calculator.add('//|\n10|20|30'), equals(60));
+    });
   });
 }