Fix Vertex AI Global Endpoint URL Construction

[NielsKSchjoedt]

Problem

When configuring RubyLLM with vertexai_location: "global", the VertexAI provider incorrectly constructs the API URL as https://global-aiplatform.googleapis.com/v1beta1. This hostname does not exist, causing requests to fail with a 404 error.

According to the Google Cloud Vertex AI documentation, the global endpoint should use the base hostname https://aiplatform.googleapis.com/v1beta1 without a location prefix.

Solution

Updated RubyLLM::Providers::VertexAI#api_base to handle "global" as a special case:

• Global: Returns https://aiplatform.googleapis.com/v1beta1
• Regional: Continues to return https://{location}-aiplatform.googleapis.com/v1beta1 (e.g., us-central1)

Changes

• Modified lib/ruby_llm/providers/vertexai.rb to conditionally build the base URL.
• Added spec/ruby_llm/providers/vertexai_spec.rb to verify the URL construction logic for both global and regional configurations.

Verification

• Added unit tests confirming correct URL generation for global vs us-central1 locations.