feat: support bearer token auth for google-vertex-anthropic

[avarayr]

Issue for this PR

Closes#14175

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

The google-vertex-anthropic loader now accepts a bearer token via VERTEX_ANTHROPIC_TOKEN env var or the /auth flow. When present, it injects a fake authClient into googleAuthOptions so google-auth-library never tries to call Google's auth endpoints. This lets you use Vertex-compatible proxies that handle their own auth.

Same pattern as the existing amazon-bedrock bearer token support.

Also fixes the env var fallback chains (project, location, token) to use || instead of ??, since Env.get() returns "" for unset vars — ?? would keep the empty string and skip the fallback.

How did you verify your code works?

Tested locally against a Vertex-compatible proxy using a bearer token.

Screenshots / recordings

N/A — no UI changes.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

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[R44VC0RP]

/review

[github-actions]

lgtm

[TwIStOy]

Thanks for working on this feature! It's really helpful!
What's the status of this PR?

[pnancarrow]

we really need this feature in order to use anthropic models on vertex with opencode. anything I could do to help push it forward?

I also propose extending this as well: avarayr#1

[rekram1-node]

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[aajisaka]

As this PR is closed, I've created a PR to resolve this issue (+ internal proxy support). Can someone review #32938?