Unify cache functions to work with both GGUF and HuggingFace caches

- Updated get_cached_model_path() to check both coderai and HF caches
- Updated download_model() to handle both URLs and HF model IDs automatically
- Made download_huggingface_model() consistent with unified API
- Updated module docstring to reflect unified cache functionality
- All cache functions now work seamlessly with both cache types