## What this does

This PR adds xAI as a model `Provider`, leveraging the xAI OpenAI
compatible API endpoints where possible.



## Type of change

- [x] Bug fix
- [ ] New feature
- [ ] Breaking change
- [x] Documentation
- [ ] Performance improvement

## Scope check

- [x] I read the [Contributing
Guide](https://github.com/crmne/ruby_llm/blob/main/CONTRIBUTING.md)
- [x] This aligns with RubyLLM's focus on **LLM communication**
- [x] This isn't application-specific logic that belongs in user code
- [x] This benefits most users, not just my specific use case

## Quality check

- [x] I ran `overcommit --install` and all hooks pass
- [x] I tested my changes thoroughly
- [x] I updated documentation if needed
- [x] I didn't modify auto-generated files manually (`models.json`,
`aliases.json`)

## API changes

- [ ] Breaking change
- [ ] New public methods/classes
- [ ] Changed method signatures
- [x] No API changes

## Related issues

<!-- Link issues: "Fixes #123" or "Related to #123" -->

---------

Co-authored-by: Carmine Paolino <carmine@paolino.me>