[tests] refactor test_models_unet_controlnetxs.py to use modular testing mixins
Refactor UNetControlNetXSModel tests to follow the modern testing
pattern with separate classes for core, memory, training, and LoRA.
Specialized tests (from_unet, freeze_unet, forward_no_control,
time_embedding_mixing) remain in the core test class.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>