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.
We often send only one field (away or status_text)
to be updated.
So we have to make our schema support optional
keys.
As a result of the more flexible schema, we no
longer need to exempt the node fixtures from
our schema checks.