[FEATURE]: Support per-provider Google credentials for Vertex AI #19930

Both google-vertex and google-vertex-anthropic currently resolve authentication from Application Default Credentials (ADC). There is no way to configure separate GCP credentials per provider.

This matters when Gemini and Anthropic models are available under different GCP projects or service accounts. For example, one account has Gemini access but not Anthropic, and a second account has Anthropic access (via Model Garden) but not Gemini.

Proposed change: Pass the googleAuthOptions field from provider.options through to the AI SDK. The @ai-sdk/google-vertex package already accepts this field natively (it maps to google-auth-library's GoogleAuthOptions), so no SDK changes are needed.

This enables config like:

{
  "provider": {
    "google-vertex": {
      "options": {
        "project": "gemini-project",
        "googleAuthOptions": { "keyFilename": "/path/to/gemini-sa.json" }
      }
    },
    "google-vertex-anthropic": {
      "options": {
        "project": "anthropic-project",
        "googleAuthOptions": { "keyFilename": "/path/to/anthropic-sa.json" }
      }
    }
  }
}

The change is fully backward-compatible: when googleAuthOptions is absent, both providers continue to use ADC exactly as they do today.

Secondary fix: google-vertex-anthropic's custom loader currently ignores provider.options entirely (the function signature is async () => instead of async (provider) =>). This means config-level project/location values are silently dropped. The proposed change fixes this as well.