feat(plugins/google-genai/vertexai): Updates to google-genai/vertexai to support apiVersion and multi-regional locations

[ecaroth]

VertexAI (renamed to Enterprise Agent Platform by google) supports multi-regional endpoints that have different base location URLs than were currently supported. This PR addresses support for that, and also allows specifically setting the api version (to v1 or v1beta1 ... was hardcoded to v1beta1 before)

Checklist (if applicable):

• PR title is following https://www.conventionalcommits.org/en/v1.0.0/
• Tested (manually, unit tested, etc.)
• Docs updated (updated docs or a docs bug required)

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[gemini-code-assist]

Code Review

This pull request introduces support for multi-regional endpoints and configurable API versions in the Vertex AI plugin, updating URL generation, client options, and option derivation utilities alongside new unit tests. The review feedback identifies a syntax error and recommends using placeholder constants in the documentation, corrects typos in error messages and corresponding test assertions, and suggests renaming a utility function to follow standard camelCase casing.

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[NicolasMahe]

I'm also affected by this issue.

My project currently uses the global Vertex AI endpoint, but I'd like to switch to the eu multi-regional endpoint, and the current implementation doesn't appear to support it correctly. This PR would unblock my use case and allow me to use multi-region deployments.

Thanks @ecaroth for working on this — I'd definitely benefit from seeing it merged.