UpdateConfig replaces the tenant's configuration.

It replaces rather than merges, so dropping a tier is omitting its field
rather than a separate call, and switching to ACAI without the deployments
clears them. Every deployment it sets must carry an endpoint and a model; a
partial one is INVALID_ARGUMENT.

Because api_key is never read back, a tier re-sent with a new endpoint or
model must carry its key again; one whose endpoint and model are unchanged
keeps the stored key and may leave api_key empty.