feat: Add digit-based number range generation for chain sum tasks

reasoning_gym/arithmetic/chain_sum.py

@@ -20,6 +20,10 @@ class ChainSumConfig:
         assert self.max_terms >= self.min_terms, "max_terms must be >= min_terms"
         assert self.min_digits > 0, "min_digits must be positive"
         assert self.max_digits >= self.min_digits, "max_digits must be >= min_digits"
+        
+        # Validate digit ranges make sense
+        if self.min_digits > 1:
+            assert 10 ** (self.min_digits - 1) >= 1, "min_digits would result in invalid number range"
 
 
 class ChainSum:
@@ -64,8 +68,21 @@ class ChainSum:
         }
 
     def _generate_task(self, rng: Random, num_terms: int, num_digits: int) -> tuple[str, int]:
-        """Generate a chain sum task"""
-        constants = [rng.randint(0, 10**num_digits) for _ in range(num_terms)]
+        """Generate a chain sum task
+        
+        Args:
+            rng: Random number generator
+            num_terms: Number of terms in the expression
+            num_digits: Number of digits for each number
+            
+        Returns:
+            Tuple of (expression string, result integer)
+        """
+        # Generate numbers with at least min_digits
+        min_value = 10 ** (num_digits - 1)  # e.g., 100 for 3 digits
+        max_value = (10 ** num_digits) - 1   # e.g., 999 for 3 digits
+        
+        constants = [rng.randint(min_value, max_value) for _ in range(num_terms)]
         operators = [rng.choice(["+", "-"]) for _ in range(num_terms - 1)]
 
         # Build expression and compute result