The xAI /v1/images/generations endpoint expects resolution as a
literal string ('1k' or '2k'), not the numeric value ('1024').
- Change _XAI_RESOLUTIONS from a dict mapping to a validation set
- Use the resolution key directly instead of the mapped value
- Fall back to DEFAULT_RESOLUTION on invalid config values
Fixes 422 Unprocessable Entity errors when resolution was sent.
13b474c56e