Complete a multipart upload process by combining all uploaded parts into the final file

This endpoint is the final step in the multipart upload workflow:

1. Start with InitiateMultipartUploadAsync to get an uploadId
2. Upload individual parts with UploadPartAsync
3. Complete the process with this endpoint

The request requires the uploadId from the initial request and a dictionary of part numbers
mapped to their ETags (received when each part is uploaded). This allows the system to
verify all parts and combine them in the correct order.

If successful, the file will be available at the specified file path.