Describe the bug
When using the Vertex AI plugin in Genkit and specifying location: 'global', the request URL becomes https://global-aiplatform.googleapis.com, which results in a 404 error. The correct URL should be https://aiplatform.googleapis.com.

It seems that the root cause is in this line of code:

Here, when location is global, it should use https://aiplatform.googleapis.com instead of https://global-aiplatform.googleapis.com.

For reference, the official Google library handles this correctly:
https://github.com/googleapis/js-genai/blob/beef4e38c8df0c41957a3b3394aa75298c646ee2/src/_api_client.ts#L185C1-L196C4

private baseUrlFromProjectLocation(): string {
    if (
      this.clientOptions.project &&
      this.clientOptions.location &&
      this.clientOptions.location !== 'global'
    ) {
      // Regional endpoint
      return `https://${this.clientOptions.location}-aiplatform.googleapis.com/`;
    }
    // Global endpoint (covers 'global' location and API key usage)
    return `https://aiplatform.googleapis.com/`;
}

To Reproduce
Steps to reproduce the behavior.

import { genkit } from 'genkit';
import { vertexAI } from '@genkit-ai/google-genai';

const ai = genkit({
  plugins: [
    vertexAI({ location: 'global' }),
  ],
});

When attempting to list or use models with location: 'global', the request fails with a 404 error.

Expected behavior
When location is set to 'global', the request should go to https://aiplatform.googleapis.com instead of https://global-aiplatform.googleapis.com, avoiding the 404 error.

Screenshots
If applicable, add screenshots to help explain your problem.

Runtime (please complete the following information):

• OS: MacOS
• Version 14.3

** Node version

• v23.1.0