feat(provider): support per-provider Google credentials for Vertex AI

[major]

Issue for this PR

Closes #19930

Type of change

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

What does this PR do?

Passes googleAuthOptions from provider config through to the AI SDK for both
google-vertex and google-vertex-anthropic, so each provider can authenticate
with separate GCP credentials.

Changes to google-vertex loader:

• Extracts googleAuthOptions from config and spreads it into SDK options
• Passes it to new GoogleAuth(...) in the custom fetch for openai-compatible models
• Switches from getApplicationDefault() to getClient() (backward-compatible,
  falls back to ADC when no explicit credentials given)

Changes to google-vertex-anthropic loader:

• Fixes a bug where the function signature was async () =>, silently ignoring
  all config options (project, location, etc.)
• Now checks config options before env vars for project and location,
  matching the google-vertex pattern
• Passes googleAuthOptions through to SDK options

Both changes are backward-compatible. When googleAuthOptions is absent, behavior
is identical to before.

How did you verify your code works?

• bun typecheck passes (exit 0)
• bun test test/provider/provider.test.ts passes (73 pass, 0 fail) including
  3 new tests:
  • google-vertex-anthropic: respects config-level project and location
  • google-vertex: passes googleAuthOptions to provider options
  • google-vertex-anthropic: passes googleAuthOptions to provider options

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]

The following comment was made by an LLM, it may be inaccurate:

Based on my search, I found related PRs that address similar authentication topics for Google Vertex AI:

Potentially Related PRs:

1. feat: add Google Vertex AI service account JSON authentication #6287 - "feat: add Google Vertex AI service account JSON authentication"

   • Related to Google Vertex AI authentication methods

2. fix(opencode): pass OAuth scopes to GoogleAuth for Vertex AI #15110 - "fix(opencode): pass OAuth scopes to GoogleAuth for Vertex AI"

   • Also involves GoogleAuth configuration for Vertex AI

3. feat: support bearer token auth for google-vertex-anthropic #14177 - "feat: support bearer token auth for google-vertex-anthropic"

   • Addresses authentication for google-vertex-anthropic provider

These PRs touch on similar authentication concerns for Google Vertex AI providers, though they may have been merged or closed previously. However, PR #19931 is the current PR you're checking, and it appears to be the first to specifically address per-provider Google credentials with googleAuthOptions support for both providers simultaneously.

[rekram1-node]

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