test(sveta): Write tests for average function

Outcome: done
Changed: tests/test_calculator.py:1 (import line), tests/test_calculator.py:88-107 (new class)
Verified by: `make verify` passed (22 tests, OK); `python3 -m compileall -q tests/test_calculator.py` passed (no warnings)
Findings: TestAverageFunction class with 6 test methods added to tests/test_calculator.py:88-107 covering empty, single-element, positive, negative, mixed, and float input cases.
Open: none
Confidence: high - all 6 acceptance criteria tests exist, all pass, no comments/docstrings, conventions followed

Typosaurus-Run: 32dcefafeb39422b82cbd65f56833df7
Typosaurus-Node: cc947cb8b24b47139b0a1b6e10d6384a
Typosaurus-Agent: @sveta
Refs: #11

tests/test_calculator.py

@@ -3,7 +3,7 @@
 import math
 import unittest
 
-from typosaurus_sandbox.domain.calculator import add, clamp, clamp_to_byte, subtract, variance
+from typosaurus_sandbox.domain.calculator import add, average, clamp, clamp_to_byte, subtract, variance
 
 
 class TestAddFunction(unittest.TestCase):
@@ -18,6 +18,28 @@ class TestAddFunction(unittest.TestCase):
         self.assertEqual(add(-3, 5), 2)
 
 
+class TestAverageFunction(unittest.TestCase):
+
+    def test_empty_sequence_raises_value_error(self) -> None:
+        with self.assertRaises(ValueError):
+            average([])
+
+    def test_single_element(self) -> None:
+        self.assertEqual(average([5]), 5.0)
+
+    def test_positive_values(self) -> None:
+        self.assertEqual(average([1, 2, 3, 4, 5]), 3.0)
+
+    def test_negative_values(self) -> None:
+        self.assertEqual(average([-10, -20, -30]), -20.0)
+
+    def test_mixed_positive_and_negative(self) -> None:
+        self.assertEqual(average([-5, 0, 5]), 0.0)
+
+    def test_float_values(self) -> None:
+        self.assertEqual(average([1.5, 2.5, 3.0]), 7.0 / 3.0)
+
+
 class TestSubtractFunction(unittest.TestCase):
 
     def test_subtract_positive(self) -> None: