Support version-aware copy sources

[Twest2]

Summary

• parse optional versionId values from x-amz-copy-source
• allow CopyObject and UploadPartCopy to read a specific source version
• add regression coverage for version-aware copy behavior

Why

Copy requests treated /bucket/key?versionId=... as part of the object key, so copy operations could only read the latest object version.

Impact

CopyObject and UploadPartCopy can now target historical source versions in versioned buckets.

Validation

• cargo test test_copy_object_specific_source_version -- --exact
• cargo test test_upload_part_copy_specific_source_version -- --exact
• cargo test copy_object
• cargo test upload_part_copy