Function select_embedding_provider 

pub fn select_embedding_provider(
    llm_config: Option<LlmEmbeddingConfig>,
) -> Box<dyn EmbeddingProvider>

Select the best available embedding provider based on configuration.

Priority order:

1. ONNX local model if onnx_model_path points to a valid .onnx file
2. LLM API if llm_embedding config is present
3. Mock (fallback, for testing or when no backend available)