widgets: Make type checks more explicit.

Note that I don't actually convert the
checker from check_dict to check_dict_only,
because that would be a user-facing change,
but I think we can sweep a lot of things
like this after the next release.

zerver/lib/validator.py

@@ -351,7 +351,10 @@ def check_widget_content(widget_content: object) -> Dict[str, Any]:
                 ]),
             )
 
+            # We re-check "type" here just to avoid it looking
+            # like we have extraneous keys.
             checker = check_dict([
+                ('type', equals('choices')),
                 ('heading', check_string),
                 ('choices', check_choices),
             ])

zerver/tests/test_widgets.py

@@ -116,20 +116,20 @@ class WidgetContentTestCase(ZulipTestCase):
             choices=[],
         )
 
-        widget_content = ujson.dumps(
-            dict(
-                widget_type='zform',
-                extra_data=zform_data,
-            ),
+        widget_content = dict(
+            widget_type='zform',
+            extra_data=zform_data,
         )
 
+        check_widget_content(widget_content)
+
         payload = dict(
             type="stream",
             to=stream_name,
             client='test suite',
             topic='whatever',
             content=content,
-            widget_content=widget_content,
+            widget_content=ujson.dumps(widget_content),
         )
         result = self.api_post(sender, "/api/v1/messages", payload)
         self.assert_json_success(result)