RED: Test exposing single number parsing bug

- Test expects single number '5' to return 5
- Currently returns 6 (off-by-one error)
- Expected: <5>, Actual: <6>

test/string_calculator_test.dart

@@ -12,5 +12,10 @@ void main() {
     test('empty string returns 0', () {
       expect(calculator.add(''), equals(0));
     });
+
+    test('single number returns that number', () {
+      expect(calculator.add('5'), equals(5));
+      expect(calculator.add('42'), equals(42));
+    });
   });
 }