test: add unit tests for Math Expression node

Add tests for _positional_alias (a-z mapping) and execute() covering
arithmetic operations, float inputs, $sum(values), and error cases.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tests-unit/comfy_extras_test/nodes_math_test.py

@@ -0,0 +1,74 @@
+import pytest
+from collections import OrderedDict
+from unittest.mock import patch, MagicMock
+
+mock_nodes = MagicMock()
+mock_nodes.MAX_RESOLUTION = 16384
+mock_server = MagicMock()
+
+with patch.dict("sys.modules", {"nodes": mock_nodes, "server": mock_server}):
+    from comfy_extras.nodes_math import _positional_alias, MathExpressionNode
+
+
+class TestPositionalAlias:
+    def test_first_letter(self):
+        assert _positional_alias(0) == "a"
+
+    def test_last_letter(self):
+        assert _positional_alias(25) == "z"
+
+    def test_all_letters(self):
+        expected = list("abcdefghijklmnopqrstuvwxyz")
+        result = [_positional_alias(i) for i in range(26)]
+        assert result == expected
+
+
+class TestMathExpressionExecute:
+    @staticmethod
+    def _exec(expression: str, **kwargs) -> object:
+        values = OrderedDict(kwargs)
+        return MathExpressionNode.execute(expression, values)
+
+    def test_addition(self):
+        result = self._exec("a + b", a=3, b=4)
+        assert result[0] == 7
+
+    def test_subtraction(self):
+        result = self._exec("a - b", a=10, b=3)
+        assert result[0] == 7
+
+    def test_multiplication(self):
+        result = self._exec("a * b", a=3, b=5)
+        assert result[0] == 15
+
+    def test_division(self):
+        result = self._exec("a / b", a=10, b=4)
+        assert result[0] == 2.5
+
+    def test_single_input(self):
+        result = self._exec("a * 2", a=5)
+        assert result[0] == 10
+
+    def test_three_inputs(self):
+        result = self._exec("a + b + c", a=1, b=2, c=3)
+        assert result[0] == 6
+
+    def test_float_inputs(self):
+        result = self._exec("a + b", a=1.5, b=2.5)
+        assert result[0] == 4.0
+
+    def test_sum_values_array(self):
+        result = self._exec("$sum(values)", a=1, b=2, c=3)
+        assert result[0] == 6
+
+    def test_non_numeric_result_raises(self):
+        with pytest.raises(ValueError, match="must evaluate to a numeric result"):
+            self._exec("$string(a)", a=42)
+
+    def test_error_message_includes_expression(self):
+        with pytest.raises(ValueError, match="'\\$string\\(a\\)'"):
+            self._exec("$string(a)", a=42)
+
+    def test_boolean_result_raises(self):
+        with pytest.raises(ValueError, match="got bool"):
+            self._exec("a > b", a=5, b=3)