fix(sse): gate the MinIO data-key trait method behind rio-v2

The method's only call site sits in the rio-v2 branch of the managed read path, so a build without that feature carried a trait method nothing could reach — a warning under default features and, with -D warnings, a hard failure of the sftp lane. The declaration now carries the same gate its implementation and its sibling decrypt_legacy_sse_dek already had.

Verified against the lane that caught it (cargo clippy -p rustfs --features sftp --all-targets -- -D warnings, clean), plus the default build and the rio-v2 interop suite (4 passed).

Refs rustfs/backlog#1638.

rustfs/src/storage/sse.rs

@@ -2719,6 +2719,7 @@ pub trait SseDekProvider: Send + Sync {
     /// Defaults to refusing: only a provider holding the MinIO master secret
     /// can serve these, and a provider that cannot must fail rather than fall
     /// back to a decoder that would misread the bytes.
+    #[cfg(feature = "rio-v2")]
     async fn decrypt_minio_sse_dek(
         &self,
         _encrypted_dek: &[u8],