Support for global and multi-region endpoints

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Update Summary: Vertex AI provider should support global and multi-region Gemini AI endpoints
Check-in Date: MM/DD/YYYY
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Problem/Motivation

Google's Gemini 3 models are only available to global and multi-region locations like us and eu right now. This means that the regular locations like 'us-central1' or 'us-west1', etc. cannot be used with the newer models.

For this module that causes 404 not founds because the URL for global and multi-region locations is different from the other location URLs. From their documentation https://docs.cloud.google.com/gemini-enterprise-agent-platform/resources...

Note: When using the global endpoint, use https://aiplatform.googleapis.com/v1/projects/${GOOGLE_CLOUD_PROJECT}/locations/global/publishers/google/models/${MODEL_ID}:generateContent instead of a service endpoint prepended with https://${GOOGLE_CLOUD_LOCATION}-aiplatform.googleapis.com.

For the us and eu multi-regions we have these URLs:

• https://aiplatform.us.rep.googleapis.com
• https://aiplatform.eu.rep.googleapis.com

Proposed resolution

Adjust the hardcoded endpoint URLs to incorporate global and multi-region options.

Hardcoded location endpoint https://git.drupalcode.org/project/ai_provider_google_vertex/-/blob/2.x/...

Target date or deadline

Remaining tasks

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