GREEN: Implement feature to ignore numbers > 1000

- Added .where((n) => n <= 1000) filter
- Numbers greater than 1000 now excluded from sum
- Tests pass: '2,1001' returns 2, '1000,1001,2' returns 1002

lib/string_calculator.dart

@@ -37,6 +37,7 @@ class StringCalculator {
         .split(delimiter)
         .where((s) => s.isNotEmpty)
         .map((s) => int.parse(s))
+        .where((n) => n <= 1000) // Filter out numbers > 1000
         .toList();
 
     // Bug 3: Off-by-one in summation