RED: Test exposing comma-delimited summation bug

- Test expects '1,2' to return 3
- Currently returns 1 (missing last element)
- Summation loop has off-by-one: for (i < length - 1)
- Expected: <3>, Actual: <1>
- Expected: <6>, Actual: <3>

test/string_calculator_test.dart

@@ -17,5 +17,15 @@ void main() {
       expect(calculator.add('5'), equals(5));
       expect(calculator.add('42'), equals(42));
     });
+
+    test('two comma-delimited numbers return sum', () {
+      expect(calculator.add('1,2'), equals(3));
+      expect(calculator.add('10,20'), equals(30));
+    });
+
+    test('multiple comma-delimited numbers return sum', () {
+      expect(calculator.add('1,2,3'), equals(6));
+      expect(calculator.add('5,10,15,20'), equals(50));
+    });
   });
 }