check_schema: Ignore operation field in schema validation for openapi.

In the openapi specs, the update_message_flags event is documented as
having a `operation` (deprecated) field, alongside the modern `op`.

This causes check_schemas warnings like this:

    NEED SCHEMA to match OpenAPI update_message_flags_add_add_event
    NEED SCHEMA to match OpenAPI update_message_flags_remove_remove_event

as check_schemas uses both `op` and `operation` for constructing the
event name.

Being deprecated (and really only still there for
backwards-compatibility with the original error of having it present),
`operation` will be removed eventually, therefore we can safely
ignore it from being used in openapi schema validation.

Part of #17568.

tools/check-schemas

@@ -184,9 +184,8 @@ def validate_openapi_against_event_schema() -> None:
 
     for sub_node in node:
         name = sub_node["properties"]["type"]["enum"][0]
-        for key in ["op", "operation"]:
-            if key in sub_node["properties"]:
-                name += "_" + sub_node["properties"][key]["enum"][0]
+        if "op" in sub_node["properties"]:
+            name += "_" + sub_node["properties"]["op"]["enum"][0]
 
         name += "_event"