🤖 Provider model changes - xai

**Provider:** `xai`
**Type:** New provider

### New models
- `grok-2-vision-1212`
- `grok-3`
- `grok-3-mini`
- `grok-4-0709`
- `grok-4-1-fast-non-reasoning`
- `grok-4-1-fast-reasoning`
- `grok-4-fast-non-reasoning`
- `grok-4-fast-reasoning`
- `grok-code-fast-1`
- `grok-2-image-1212`
- `grok-imagine-image`
- `grok-imagine-image-pro`
- `grok-imagine-video`
