Add support for Gemini EU multi-region REP endpoint and stable Gemini 3.x model IDs

[photonn]

Name and Version

0.35.0

What is the problem this feature will solve?

We are trying to configure the Vertex AI adapter for the new Gemini EU datazone / multi-region models:

gemini-3.5-flash
gemini-3.1-flash-lite

Google’s Gemini Enterprise Agent Platform documentation lists both models as available in the eu multi-region location. The multi-region endpoint documentation says the EU endpoint should be:

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

However, with ai-dial-adapter-vertexai 0.35.0, configuring the DIAL upstream with:

{ "extraData": { "region": "eu" } }

appears to make the adapter pass location="eu" to google-genai. In google-genai==1.64.0, that resolves to:

https://eu-aiplatform.googleapis.com/

rather than the documented REP endpoint:

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

As a result, the adapter does not seem able to target the required EU multi-region REP endpoint via DIAL configuration.

Current Behavior
The adapter supports passing a region through upstream extraData.region, but region: "eu" is resolved by the Google Gen AI SDK to https://eu-aiplatform.googleapis.com/.

The adapter also does not appear to natively include the stable model IDs:

gemini-3.5-flash
gemini-3.1-flash-lite

Only preview-compatible IDs appear to be available, such as:

gemini-3-flash-preview
gemini-3.1-flash-lite-preview

Using compatible_model_id can bypass adapter model compatibility checks, but it does not solve the endpoint hostname problem.

What is the feature you are proposing to solve the problem?

Please add support for one of the following:

1. Check if new versions of google-genai address this issue, and if yes, implement their proposed solution.

2. Native mapping for multi-region REP locations, for example:

   eu -> https://aiplatform.eu.rep.googleapis.com
   us -> https://aiplatform.us.rep.googleapis.com

3. A configurable upstream base_url / endpoint option for Gemini requests, for example:
   { "extraData": { "region": "eu", "base_url": "https://aiplatform.eu.rep.googleapis.com" } }

Also add native support for the stable model IDs:
gemini-3.5-flash
gemini-3.1-flash-lite

Reference: https://docs.cloud.google.com/gemini-enterprise-agent-platform/resources/locations?hl=es-419#multi-region_endpoints

What alternatives have you considered?

None

[AlexFoxalt]

Hello @photonn,

The requested changes have been delivered to the development branch.

Regards,
Alex