refactor: ui: Patch *_box_views in OPEN_DRAFT tests.

This commit assigns patches `stream_box_view` and `private_box_view`
using `mocker.patch.object` to maintain type consistency since their
mocked nature's assert methods are being called.

tests/ui/test_ui.py

@@ -361,17 +361,21 @@ def test_keypress_OPEN_DRAFT(self, view, mocker, draft, key, widget_size):
             "foo@zulip.com": {"user_id": 1},
             "bar@gmail.com": {"user_id": 2},
         }
+        mocked_stream_box_view = mocker.patch.object(view.write_box, "stream_box_view")
+        mocked_private_box_view = mocker.patch.object(
+            view.write_box, "private_box_view"
+        )
 
         size = widget_size(view)
         view.keypress(size, key)
 
         if draft:
             if draft["type"] == "stream":
-                view.write_box.stream_box_view.assert_called_once_with(
+                mocked_stream_box_view.assert_called_once_with(
                     caption=draft["to"], title=draft["subject"], stream_id=10
                 )
             else:
-                view.write_box.private_box_view.assert_called_once_with(
+                mocked_private_box_view.assert_called_once_with(
                     emails=draft["to"], recipient_user_ids=[1, 2]
                 )