docs: sync dewey Vertex AI global region endpoint support

[jflowers]

What Changed

Dewey's Vertex AI providers now support region: global in config.yaml. When set to global, the endpoint URL uses aiplatform.googleapis.com (no region prefix) to route requests to the nearest available region. Previously, only regional endpoints (e.g., us-east5, us-central1) were supported.

This was a bug fix — the global value was accepted but produced an invalid URL (global-aiplatform.googleapis.com).

Website Pages Affected

• Provider configuration reference (the region field documentation)
• Vertex AI setup guide (if one exists from docs: sync dewey pluggable provider configuration documentation #113)
• Any config examples showing region: should mention global as a valid option

What Needs Updating

• Document that region accepts either a GCP region name (e.g., us-east5) or global
• Explain that global routes to the nearest available region
• Update any config.yaml examples to show both options

Related

• Supersedes the region portion of docs: sync dewey pluggable provider configuration documentation #113 (pluggable provider docs)
• Branch: opsx/vertex-global-region-url in unbound-force/dewey