Can't use Vertex with Anthropic

[sxlijin]

The Anthropic models are not accessible through the "standard" Vertex APIs: GCP actually only exposes them using the Anthropic APIs.

As a result, since we go through the standard Vertex APIs, the only way that BAML users can use Vertex with Anthropic today is by running a separate proxy server to translate their Vertex API calls into Anthropic API calls, which raises a number of issues (operating another server, figuring out buffering/proxies for streams, etc)

BAML users should instead just be able to use "vertex/anthropic:claude-35-sonnet-..." (whatever the version specifier is).

We currently have at least two users blocked on this, see discord