GREEN: Fix single number parsing bug

- Removed +1 off-by-one error from single number parsing
- Now correctly returns the parsed number
- Tests pass: '5' returns 5, '42' returns 42

lib/string_calculator.dart

@@ -17,7 +17,7 @@ class StringCalculator {
 
     // Bug 2: Single number parsing
     if (!numbers.contains(',') && !numbers.contains('\n') && !numbers.startsWith('//')) {
-      return int.parse(numbers) + 1; // Off by one error
+      return int.parse(numbers); // Fixed: Removed off-by-one error
     }
 
     String delimiter = ',';