test: cover add()-to-list and upsert()-on-scalar-root routing errors

Add two missing cases to TestRoutingErrorBehaviour:

- test_add_through_list_raises_routing_error: add() through a list node
  was untested; only the scalar-node case existed.
- test_upsert_scalar_root_raises_routing_error: exercises the
  _create_at(parent_keys=(), ...) branch on a non-empty document and
  pins the '<root>' token in RoutingError messages.

Author: nathanjmcdougall

tests/test_errors.py

@@ -82,3 +82,13 @@ def test_routing_error_is_catchable_as_patch_error(self):
         doc = Document("a: scalar\n")
         with pytest.raises(PatchError):
             doc.upsert("a", "b", value="foo")
+
+    def test_add_through_list_raises_routing_error(self):
+        doc = Document("items:\n  - a\n")
+        with pytest.raises(RoutingError, match="non-mapping node at items"):
+            doc.add("items", key="child", value="foo")
+
+    def test_upsert_scalar_root_raises_routing_error(self):
+        doc = Document("just_a_scalar\n")
+        with pytest.raises(RoutingError, match=r"non-mapping node at <root>"):
+            doc.upsert("y", value="v")