filer: prefix bucket default collection, assign copy collection explicitly
S3 server-side copy paths (CopyObject/UploadPartCopy) assign volumes with an empty collection and relied on the filer's bucket fallback. The fallback used the raw bucket name, while PutObject uses getCollectionName(bucket) = <filerGroup>_<bucket>, so copies landed in a second, unprefixed collection.
Prefix the filer's bucket default with the filer group, and have the S3 copy path pass that collection explicitly, like PutObject. Direct filer writes keep path-rule precedence, so this only aligns S3 copy with S3 put.
Adds regression tests for the fallback, explicit collection precedence, and the S3 copy collection resolver.
