fix VolumeTierMoveDatFromRemote to close remote dat backend after download

Go calls v.DataBackend.Close() and sets DataBackend=nil after removing
the remote file reference. Without this, the stale remote backend
state lingers and reads may not discover the newly downloaded local
.dat file until server restart.

seaweed-volume/src/server/grpc_server.rs

@@ -3029,6 +3029,10 @@ impl VolumeServer for VolumeGrpcService {
                                     vid, e
                                 )));
                             }
+
+                            // Close old remote backend (matches Go: v.DataBackend.Close(); v.DataBackend = nil)
+                            // This forces the next read to discover and open the local .dat file.
+                            vol.close_remote_dat_backend();
                         }
                     }
                 }

seaweed-volume/src/storage/volume.rs

@@ -2090,6 +2090,12 @@ impl Volume {
         self.dat_file = None;
     }
 
+    /// Close the remote dat file backend (matches Go's v.DataBackend.Close(); v.DataBackend = nil).
+    /// Called after tier-download when the remote backend is being replaced by local storage.
+    pub fn close_remote_dat_backend(&mut self) {
+        self.remote_dat_file = None;
+    }
+
     /// Path to .vif file.
     fn vif_path(&self) -> String {
         format!("{}.vif", self.data_file_name())