Add Claude models (Anthropic) for Vertex AI provider

Currently, the Vertex AI integration only shows Gemini models in the model picker. Google Cloud now offers Claude models (Anthropic) as managed models on Vertex AI, including:

• claude-sonnet-4@20250514

• claude-opus-4@20250514

• claude-3-5-haiku@20241022

It would be great to either:

1. Add these to the Vertex AI model dropdown, or

2. Allow a custom model ID field for Vertex AI (so users can specify any model)

This would be especially useful for those using Google Cloud credits who want to access Claude through Vertex rather than a separate Anthropic API key.

Thanks!