AWS S3 behavior: when calling GetObject or HeadObject with the PartNumber
query parameter, the ETag header should still return the complete object's
ETag (e.g., 'abc123-4' for a 4-part multipart upload), not the individual
part's ETag.
The previous implementation incorrectly overrode the ETag with the part's
ETag, causing test_multipart_get_part to fail.
This fix removes the ETag override logic while keeping:
- x-amz-mp-parts-count header (correct)
- Content-Length adjusted to part size (correct)
- Range calculation for part boundaries (correct)
a5ab05ec03