feat: Add iteration support to ArithmeticDataset with size-based termination

reasoning_gym/arithmetic/basic_arithmetic.py

@@ -141,6 +141,19 @@ class ArithmeticDataset:
         expression = " ".join(expression_parts)
         return expression, result
 
+    def __iter__(self):
+        """Make the dataset iterable"""
+        self._current_idx = 0
+        return self
+        
+    def __next__(self):
+        """Get next item in iteration"""
+        if self._current_idx >= self.config.size:
+            raise StopIteration
+        item = self[self._current_idx]
+        self._current_idx += 1
+        return item
+
     def _format_question(self, expression: str) -> str:
         """Format the expression according to config style"""
         if self.config.format_style == "simple":