feat: Add ChainSum class for generating simple arithmetic tasks

tests/test_chain_sum.py

@@ -0,0 +1,51 @@
+import pytest
+from reasoning_gym.arithmetic import ChainSum, ChainSumConfig
+
+
+def test_chain_sum_config_validation():
+    """Test that invalid configs raise appropriate errors"""
+    with pytest.raises(AssertionError):
+        config = ChainSumConfig(min_terms=0)
+        config.validate()
+    
+    with pytest.raises(AssertionError):
+        config = ChainSumConfig(min_terms=3, max_terms=2)
+        config.validate()
+
+
+def test_chain_sum_deterministic():
+    """Test that dataset generates same items with same seed"""
+    config = ChainSumConfig(seed=42, size=10)
+    dataset1 = ChainSum(config)
+    dataset2 = ChainSum(config)
+    
+    for i in range(len(dataset1)):
+        assert dataset1[i] == dataset2[i]
+
+
+def test_chain_sum_items():
+    """Test basic properties of generated items"""
+    config = ChainSumConfig(
+        min_terms=2,
+        max_terms=4,
+        min_digits=1,
+        max_digits=2,
+        size=100,
+        seed=42
+    )
+    dataset = ChainSum(config)
+    
+    for i in range(len(dataset)):
+        item = dataset[i]
+        assert isinstance(item, dict)
+        assert "question" in item
+        assert "answer" in item
+        assert "metadata" in item
+        
+        # Verify only + and - are used
+        expression = item["metadata"]["expression"]
+        assert all(op in ["+", "-", " "] or op.isdigit() for op in expression)
+        
+        # Verify the answer matches the expression
+        answer = eval(expression)  # Safe here as we control the expression
+        assert str(answer) == item["answer"]