processExplicitDirectory appended a directory-key object as a version
without checking it against the prefix. A versioned listing descends
through ancestor markers to reach a deeper prefix, so every ancestor
(Veeam/, Veeam/Backup/, ...) leaked into Versions even though none of
them match the prefix - which makes Veeam's immutable repository scan
abort on an unexpected key. Guard on the prefix so only keys at or under
it surface, matching ListObjectsV2 and AWS.