Add google-vertex-openapi provider for third-party models (GLM)

[leehack]

Problem

The existing google-vertex provider uses the official SDK and only supports Google's native models (Gemini). Third-party models like GLM-4.7 from ZhipuAI are available on Vertex AI but require access through the OpenAPI-compatible endpoint with different authentication (gcloud ADC or service account instead of the official SDK).

Requested Solution

Add a new google-vertex-openapi provider that:

1. Uses the OpenAPI-compatible endpoint on Vertex AI
2. Supports Application Default Credentials (ADC) via google-auth-library
3. Works with "gcloud auth application-default login" or service account authentication
4. Allows configuring project ID and location via config or environment variables
5. Is configurable for third-party models like GLM-4.7

Use Cases

• Users with GLM-4.7 models deployed on Vertex AI
• Other third-party models available via Vertex Model Garden
• Users who prefer gcloud ADC authentication over service account management

Alternative Solutions Considered

• Use the existing google-vertex provider: Does not work for third-party models
• Add models directly to models.dev API: Requires API access
• Create an auth plugin: Plugins cannot add new providers, only modify existing ones

Additional Context

This follows the existing custom loader pattern in provider.ts.

[github-actions]

This issue might be a duplicate of existing issues. Please check:

• Unable to access GLM 4.7 on Google Vertex AI #9909: Unable to access GLM 4.7 on Google Vertex AI

This issue describes the exact same problem - users trying to access GLM 4.7 models on Vertex AI through the OpenAPI-compatible endpoint. The current google-vertex provider doesn't support third-party models like GLM, and #9909 is reporting that attempting to use GLM 4.7 results in a "Not Found" error. Your proposed google-vertex-openapi provider would directly solve that issue.

Feel free to ignore if your feature request has a different scope than the existing issue.