refactor(semantic_search_client): Remove ONNX embedder and use Candle as default

- Removed ONNX embedder implementation and fastembed dependency
- Updated conditional compilation to use Candle on all platforms except Linux ARM
- Fixed duplicate implementations in error.rs
- Updated benchmark tests to remove ONNX references

🤖 Assisted by [Amazon Q Developer](https://aws.amazon.com/q/developer)

Author: kennvene-aws