Improve model discovery: skip unfound models, add deep search for variants

- Skip models not found on HuggingFace instead of adding with defaults
- Add deep search for model variants from known organizations
- Search organizations: Alpha-VLLM, stepvideo, hpcai-tech, tencent,
  rhymes-ai, THUDM, genmo, Wan-AI, stabilityai, black-forest-labs
- Remove non-existent models from known_large_models list
- Better error handling for model validation