CVE-2026-39360: RustFS has an authorization bypass in multipart UploadPartCopy enables cross-bucket object exfiltration

RustFS contains a missing authorization check in the multipart copy path (UploadPartCopy). A low-privileged user who cannot read objects from a victim bucket can still exfiltrate victim objects by copying them into an attacker-controlled multipart upload and completing the upload.

This breaks tenant isolation in multi-user / multi-tenant deployments.