Update Google GenAI integration to support Vertex

[anakin87]

Google GenAI API supports both Gemini Developer API and Vertex AI API.

The way our integration creates the client makes it compatible with Gemini Developer API only (like our old Google AI integration).

Based on Google GenAI API docs, we should allow more parameters for client creation in order to support the Vertex AI API.

The Generative AI module in the Vertex AI SDK is deprecated and will no longer be available after June 24, 2026.

[anakin87]

Reopening because we should also:

• document the different authentication options in our docs
• update the integration page similarly