fix(anthropic): anthropic with vertex works with service account json keys

[Cali0707]

• I have read CONTRIBUTING.md.
• I have created a discussion that was approved by a maintainer (for new features). Not needed, this is a bug fix

The bug

Currently, when using this package with an anthropic model from vertex, it fails when trying to use service account json keys for auth. This is a common auth setup for e.g. CI/CD pipelines, or where orgs restrict user based ADC/API key creation.

In particular, this is the error that I was seeing (redacted project/region details):

"error":"Post \"https://<region>-aiplatform.googleapis.com/v1/projects/<projectID>/locations/<region>/publishers/anthropic/models/claude-opus-4-6:streamRawPredict\": oauth2: cannot fetch token: 400 Bad Request\nResponse: {\"error\":\"invalid_scope\",\"error_description\":\"Invalid OAuth scope or ID token audience provided.\"}"

Proposed fix

This is not an issue when using the google provider with vertex directly, as that gets the credentials through the google genai package, which sets the scope itself: https://github.com/googleapis/go-genai/blob/37134ea8c0c17d262c26ed46e31ada75406dff02/client.go#L366

All I changed is also setting this scope when using the vertex option in the anthropic provider