Switch from ResponseHeadersValidationError to RuntimeError

Response instances cannot be validated, and hence it is considered a
programming error (RuntimeError) to try.

It isn't clear why b5c610b choose a
ResponseHeadersValidationError, but it doesn't make sense as the
exception type.

src/quart_schema/validation.py

@@ -223,7 +223,7 @@ async def wrapper(*args: Any, **kwargs: Any) -> Any:
 
             if isinstance(value, (Response, WerkzeugResponse)):
                 if status == status_code:
-                    raise ResponseHeadersValidationError()
+                    raise RuntimeError("Cannot validate Response instance")
                 else:
                     return result
 

tests/test_validation.py

@@ -211,6 +211,20 @@ async def item() -> ResponseReturnValue:
     assert response.status_code == 302
 
 
+async def test_response_validation_of_response() -> None:
+    app = Quart(__name__)
+    QuartSchema(app)
+
+    @app.route("/")
+    @validate_response(PyItem)
+    async def item() -> ResponseReturnValue:
+        return Response(200)
+
+    test_client = app.test_client()
+    response = await test_client.get("/")
+    assert response.status_code == 500
+
+
 @pytest.mark.parametrize(
     "return_value, status",
     [