Fix Vertex AI Global Endpoint URL Construction (#553)

### 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](https://docs.cloud.google.com/vertex-ai/generative-ai/docs/learn/locations#global-endpoint),
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
- [x] Added unit tests confirming correct URL generation for `global` vs
`us-central1` locations.

Co-authored-by: Carmine Paolino <carmine@paolino.me>

Author: NielsKSchjoedt

lib/ruby_llm/providers/vertexai.rb

@@ -16,7 +16,11 @@ def initialize(config)
       end
 
       def api_base
-        "https://#{@config.vertexai_location}-aiplatform.googleapis.com/v1beta1"
+        if @config.vertexai_location == 'global'
+          'https://aiplatform.googleapis.com/v1beta1'
+        else
+          "https://#{@config.vertexai_location}-aiplatform.googleapis.com/v1beta1"
+        end
       end
 
       def headers

spec/ruby_llm/providers/vertexai_spec.rb

@@ -0,0 +1,24 @@
+require 'spec_helper'
+
+RSpec.describe RubyLLM::Providers::VertexAI do
+  let(:config) { double('config', vertexai_location: location, vertexai_project_id: 'test-project') }
+  subject { described_class.new(config) }
+
+  describe '#api_base' do
+    context 'when location is global' do
+      let(:location) { 'global' }
+
+      it 'uses the correct api_base without location prefix' do
+        expect(subject.api_base).to eq('https://aiplatform.googleapis.com/v1beta1')
+      end
+    end
+
+    context 'when location is not global' do
+      let(:location) { 'us-central1' }
+
+      it 'uses the correct api_base with location prefix' do
+        expect(subject.api_base).to eq('https://us-central1-aiplatform.googleapis.com/v1beta1')
+      end
+    end
+  end
+end