test(sveta): Write API integration tests for calculator endpoints

Outcome: done
Changed: tests/test_api.py:53-68,75-85,90-98 — added clamp low>high, missing-field, and subtract missing-field tests; src/typosaurus_sandbox/presentation/api/v1/calculator.py:2,51-55 — added HTTPException import and ValueError catch in calculate_clamp
Verified by: `make verify` — exit 0, 47 tests OK, zero warnings, zero compile errors
Findings:
- tests/test_api.py now contains 22 integration tests covering all five endpoints with success, validation errors (missing fields, wrong types), and the clamp low>high ValueError boundary.
- clamp low>high error is returned as 422 via HTTPException in the route handler, matching the acceptance criterion.
- All 47 tests (25 unit + 22 integration) pass with zero warnings.
Open: none
Confidence: high — all acceptance criteria addressed, all tests pass, verification gate passed

Typosaurus-Run: d4f7c095ea9d49c69663ae6d01a21513
Typosaurus-Node: 9fe1a5bb3c164f6eb359366ca55b1067
Typosaurus-Agent: @sveta
Refs: #15

tests/test_api.py

@@ -54,6 +54,14 @@ class TestCalculatorSubtractEndpoint(unittest.TestCase):
         response = client.post("/api/v1/calculator/subtract", json={"left": 10, "right": None})
         self.assertEqual(response.status_code, 422)
 
+    def test_subtract_missing_left_returns_422(self) -> None:
+        response = client.post("/api/v1/calculator/subtract", json={"right": 3})
+        self.assertEqual(response.status_code, 422)
+
+    def test_subtract_missing_right_returns_422(self) -> None:
+        response = client.post("/api/v1/calculator/subtract", json={"left": 10})
+        self.assertEqual(response.status_code, 422)
+
 
 class TestCalculatorClampEndpoint(unittest.TestCase):
 
@@ -76,6 +84,18 @@ class TestCalculatorClampEndpoint(unittest.TestCase):
         response = client.post("/api/v1/calculator/clamp", json={"value": "x", "low": 0, "high": 10})
         self.assertEqual(response.status_code, 422)
 
+    def test_clamp_low_greater_than_high_returns_422(self) -> None:
+        response = client.post("/api/v1/calculator/clamp", json={"value": 5, "low": 10, "high": 0})
+        self.assertEqual(response.status_code, 422)
+
+    def test_clamp_missing_low_returns_422(self) -> None:
+        response = client.post("/api/v1/calculator/clamp", json={"value": 5, "high": 10})
+        self.assertEqual(response.status_code, 422)
+
+    def test_clamp_missing_high_returns_422(self) -> None:
+        response = client.post("/api/v1/calculator/clamp", json={"value": 5, "low": 0})
+        self.assertEqual(response.status_code, 422)
+
 
 class TestCalculatorClampToByteEndpoint(unittest.TestCase):