Describe the bug

Gemini 3 models (gemini-3-pro-preview, gemini-3-flash-preview) require the global endpoint on Vertex AI. Setting GCP_LOCATION=global does not work because the provider constructs an invalid URL.

In crates/goose/src/providers/gcpvertexai.rs:

let host = format!("https://{}-aiplatform.googleapis.com", location);

This produces https://global-aiplatform.googleapis.com instead of https://aiplatform.googleapis.com.

To Reproduce

1. Set config:

   GOOSE_PROVIDER: gcp_vertex_ai
   GOOSE_MODEL: gemini-3-pro-preview
   GCP_LOCATION: global

2. Run goose session
3. Send any message
4. Request fails

Expected behavior

When GCP_LOCATION=global, use endpoint https://aiplatform.googleapis.com with locations/global in the path.

Please provide the following information

• OS & Arch: macOS 26.1 arm64
• Interface: CLI
• Version: 1.17.0
• Extensions enabled: Developer Tools
• Provider & Model: GCP Vertex AI – gemini-3-pro-preview

Additional context

Regional endpoint returns 404:

curl -X POST "https://us-central1-aiplatform.googleapis.com/.../gemini-3-pro-preview:generateContent"
# 404 NOT_FOUND

Global endpoint works:

curl -X POST "https://aiplatform.googleapis.com/v1/projects/{project}/locations/global/publishers/google/models/gemini-3-pro-preview:generateContent"
# Success