feat(openai): add xAI Grok Imagine Image 2.0 image model support

- Added `grok-imagine-image-2.0` as a first-class xAI image base model across validation, canonicalization, and routing checks.
- Registered the model in built-in model definitions so it appears in model metadata.
- Updated image model allowlists and request validation error messaging to include the new model.
- Marked the new model as hidden in client visibility override handling.