Parse the Checksum* elements of each completed part and enforce what AWS
does for uploads created with x-amz-checksum-algorithm: every part must
carry a checksum in the complete request (InvalidRequest when missing,
BadDigest when it differs from the stored part checksum), and an
x-amz-checksum-type header must match the upload resolved checksum type
(BadDigest). Add the issue-11401 reproduction as a regression test.