cloud_llm_client: Make `default_model` and `default_fast_model` optional (#38288)

This PR makes the `default_model` and `default_fast_model` fields
optional on the `ListModelsResponse`.

Release Notes:

- N/A
74e5b848ffMarshall Bowers committed on 9/16/2025, 10:24:03 PM· committed by GitHubparentee399eb
2 files changedLine totals unavailable