Add grok-vision-beta to xAI + Update grok-beta Features

[taowang1993]

Summary

This PR adds grok-vision-beta to the xAI provider.
It also updates the grok-beta model with more features.

Screenshots

[taowang1993]

One caveat:

In grok-vision-beta.yaml, the feature section should only include:

• agent-thought
• vision

The function/tool calling feature will work without adding

• tool-call
• multi-tool-call
• stream-tool-call

The function/tool calling feature will stop working if you add them to the feature section.
It will return an error saying something like "the model does not support function/tool calling".

However, the function/tool calling works just fine witout specifying these features in the feature section.