CompleteMultipartUpload response omits ChecksumType

[Vonng]

Expected Behavior

The CompleteMultipartUploadResult XML should carry <ChecksumType> when the object has a
checksum, as AWS S3 does. The
API reference
lists it in the response body with valid values COMPOSITE | FULL_OBJECT.

Current Behavior

The field does not exist on the response struct, so it is never emitted:

cmd/api-response.go

type CompleteMultipartUploadResponse struct {
	XMLName xml.Name `xml:"http://s3.amazonaws.com/doc/2006-03-01/ CompleteMultipartUploadResult" json:"-"`

	Location string
	Bucket   string
	Key      string
	ETag     string

	ChecksumCRC32     string `xml:"ChecksumCRC32,omitempty"`
	ChecksumCRC32C    string `xml:"ChecksumCRC32C,omitempty"`
	ChecksumSHA1      string `xml:"ChecksumSHA1,omitempty"`
	ChecksumSHA256    string `xml:"ChecksumSHA256,omitempty"`
	ChecksumCRC64NVME string `xml:",omitempty"`
}

generateCompleteMultipartUploadResponse already computes the map that contains it
(oi.decryptChecksums returns x-amz-checksum-type), it is simply dropped on the floor.

Observed with real boto3 1.43.58 against a FULL_OBJECT CRC32 upload:

complete_multipart_upload() -> ChecksumType: None          # AWS returns FULL_OBJECT
head_object()               -> ChecksumType: FULL_OBJECT   # correct

ListParts (api-response.go) and GetObjectAttributes (object-handlers.go) both return
the field correctly; only the completion response is missing it.

Impact

Presentation only - no data is affected. Clients that validate the returned checksum type
after completing an upload see a null value.

Context

Found while root-causing #31.