- Added `_download_files_parallel` function to enable concurrent downloading of files from Google Drive, improving efficiency in document processing.
- Introduced `_download_and_index` function to handle the parallel downloading and indexing phases, streamlining the overall workflow.
- Updated `_index_full_scan` and `_index_with_delta_sync` methods to utilize the new parallel downloading functionality, enhancing performance.
- Added unit tests to validate the new parallel downloading and indexing logic, ensuring robustness and error handling during document processing.
