Support the vertex AI global endpoint

[mboulton-fathom]

In the source code, it says:

      Vertex Supports 3 different URL formats 
      1. Standard VertexAI: e.g. https://{location}-aiplatform.googleapis.com/v1/projects/{project}/locations/{location}/publishers/google/models/{model}:streamGenerateContent
      2. Tuned model:       e.g. https://{location}-aiplatform.googleapis.com/v1/projects/{project}/locations/{location}/endpoints/{endpoint}:streamGenerateContent
      3. Express mode:      e.g. https://aiplatform.googleapis.com/v1/publishers/google/models/{model}:streamGenerateContent?key={API_KEY}

But there is actually a 4th endpoint that vertex AI supports, similar to the first listed but without the location prefix, and the second 'location' hardcoded to 'global':

https://aiplatform.googleapis.com/v1/projects/{project}/locations/global/publishers/google/models/{model}:streamGenerateContent

https://docs.cloud.google.com/vertex-ai/generative-ai/docs/learn/locations#use_the_global_endpoint

To use the global endpoint, exclude the location from the endpoint name and configure the location of the resource to global.

This is currently a blocker to using gemini 3 pro via vertex AI, AFAICT.