refactor(semantic_search_client): Remove ONNX embedder and use Candle as default (#1888)

- 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)

Co-authored-by: Kenneth Sanchez V <kennvene@amazon.com>

Author: kensave