* fix: ListObjectVersions interleave Version and DeleteMarker in sort order
Go's default xml.Marshal serializes struct fields in definition order,
causing all <Version> elements to appear before all <DeleteMarker>
elements. The S3 API contract requires these elements to be interleaved
in the correct global sort order (by key ascending, then newest version
first within each key).
This broke clients that validate version list ordering within a single
key — an older Version would appear before a newer DeleteMarker for the
same object.
Fix: Replace the separate Versions/DeleteMarkers/CommonPrefixes arrays
with a single Entries []VersionListEntry slice. Each VersionListEntry
uses a per-element MarshalXML that outputs the correct XML tag name
(<Version>, <DeleteMarker>, or <CommonPrefixes>) based on which field
is populated. Since the entries are already in their correct sorted
order from buildSortedCombinedList, the XML output is automatically
interleaved correctly.
Also removes the unused ListObjectVersionsResult struct.
Note: The reporter also mentioned a cross-key timestamp ordering issue
when paginating with max-keys=1, but that is correct S3 behavior —
ListObjectVersions sorts by key name (ascending), not by timestamp.
Different keys having non-monotonic timestamps is expected.
* test: add CommonPrefixes XML marshaling coverage for ListObjectVersions
* fix: validate VersionListEntry has exactly one field set in MarshalXML
Return an error instead of silently emitting an empty <Version> element
when no field (or multiple fields) are populated. Also clean up the
misleading xml:"Version" struct tag on the Entries field.
see https://blog.aqwari.net/xml-schema-go/
1. go get aqwari.net/xml/cmd/xsdgen
2. Add EncodingType element for ListBucketResult in AmazonS3.xsd
3. xsdgen -o s3api_xsd_generated.go -pkg s3api AmazonS3.xsd
4. Remove empty Grantee struct in s3api_xsd_generated.go
5. Remove xmlns: sed s'/http:\/\/s3.amazonaws.com\/doc\/2006-03-01\/\ //' s3api_xsd_generated.go
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