### Check for existing issues

- [x] I have searched the existing issues and checked that my issue is not a duplicate.

### What happened?

Similarly to #19138, seems that litellm forces vertex service account credentials for endpoints that only require a token.

The situation is as follows: - Regional endpoints (such as `us-central1-aiplatform.googleapis.com`) require service account authentication and token - Global (express) endpoints (such as `aiplatform.googleapis.com`) do not require authentication, in particular: - Vertex AI "standard" users can use the global endpoint without authenticating with their service account - Vertex Express users do not even have a service account at all bound to their tokens

### Steps to Reproduce

1. Make sure you are unauthenticated on Vertex AI, and you have an API key 2. Check that the global endpoint is working unauthenticated ``` curl -X POST "https://aiplatform.googleapis.com/v1/publishers/google/models/gemini-3-flash-preview:generateContent?key=<my_key>" \ -H "Content-Type: application/json" \ -d '{ "contents": [ { "role": "user", "parts": [ { "text": "Why is the sky blue?" } ] …