### Check for existing issues

- [x] I have searched the existing issues and checked that my issue is not a duplicate.

### The Feature

3 Grok models have recently been made available on Vertex AI:

+ xai/grok-4.1-fast-non-reasoning + xai/grok-4.1-fast-reasoning + xai/grok-4.20-non-reasoning

Would be great if litellm can support it. Currently trying to access these models through LiteLLM will results in following error, might be due to lack of correct mapping:

``` litellm.APIConnectionError: Value frozenset({'MALFORMED_RESPONSE', 'TOO_MANY_TOOL_CALLS', 'FINISH_REASON_UNSPECIFIED', 'SAFETY', 'LANGUAGE', 'IMAGE_SAFETY', 'MAX_TOKENS', 'PROHIBITED_CONTENT', 'OTHER', 'RECITATION', 'STOP', 'MALFORMED_FUNCTION_CALL', 'IMAGE_PROHIBITED_CONTENT', 'SPII', 'BLOCKLIST'}) has unexpected type <class 'frozenset'> at Value._GEMINI_FINISH_REASON_KEYS. Received Model Group=xai/grok-4.1-fast-non-reasoning\nAvailable Model Group Fallbacks=None ```

### Motivation, pitch

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### What part of LiteLLM is this about?

Proxy

### LiteLLM is hiring a founding backend engineer, are you interested in joining us and shipping to all our users?

No

### Twitter / LinkedIn details

_No response_