Updates the list of models being available based on
https://docs.x.ai/developers/models

From the xAI site:
<img width="750" height="169" alt="image"
src="https://github.com/user-attachments/assets/328c2743-2de7-4324-8eda-19e4b1f734e9"
/>

Closes #55883

Release Notes:

- agent: Added support for grok-4.3, grok-4.2 and removed deprecated xAI
models