refac: drop the redundant .keys() from two dict membership tests (#28859)

`x in d` and `x in d.keys()` are identical for a plain dict, so the `.keys()` call builds a throwaway view and reads as if it were doing something. Both sites operate on a plain dict: `combined` in `merge_and_sort_query_results` is a local `dict()`, and `ui_settings` comes from `UserSettings.model_dump()` where `ui` is annotated `dict | None` and is already guarded against None on the preceding line.

No behaviour change, and no measurable speedup either, so this is a readability cleanup rather than a performance one.

Sites where `.keys()` is load-bearing are left alone: the `list(d.keys())` snapshots taken before mutating during iteration, and the places where `.keys()` is the iteration or comprehension source rather than a membership test.

backend/open_webui/routers/users.py

@@ -515,7 +515,7 @@ async def update_user_settings_by_session_user(
     if (
         user.role != 'admin'
         and ui_settings is not None
-        and 'toolServers' in ui_settings.keys()
+        and 'toolServers' in ui_settings
         and not await has_permission(
             user.id,
             'features.direct_tool_servers',