Match Go VolumeDelete: propagate only_empty to delete_volume for defense in depth

The gRPC VolumeDelete handler had a pre-check for only_empty but then
passed false to store.delete_volume(), bypassing the store-level check.
Go passes req.OnlyEmpty directly to DeleteVolume. Now Rust does the same
for defense in depth against TOCTOU races (though the store write lock
makes this unlikely).

seaweed-volume/src/server/grpc_server.rs

@@ -653,7 +653,7 @@ impl VolumeServer for VolumeGrpcService {
             }
         }
         store
-            .delete_volume(vid, false)
+            .delete_volume(vid, req.only_empty)
             .map_err(|e| Status::internal(e.to_string()))?;
         Ok(Response::new(volume_server_pb::VolumeDeleteResponse {}))
     }