fix(nfs): explicitly abort the server JoinHandle on shutdown

`tokio::pin!(server_handle)` plus dropping it at function end does not
cancel a tokio task — the JoinHandle's Drop is a no-op for the underlying
task. The NFS accept loop in `nfsserve::handle_forever()` would keep
running after `mount_nfs` returned, leaking the bound TCP port until
process exit. Caught by codex review on the Windows port PR.

Now: explicit `.abort(); let _ = .await;` in the shutdown sequence.

Author: XciD

src/nfs.rs

@@ -585,6 +585,10 @@ pub async fn mount_nfs(
         }
     }
 
+    // Stop the NFS server task explicitly (dropping the JoinHandle does not
+    // cancel a tokio task — the server would keep accept()ing on its socket).
+    server_handle.abort();
+    let _ = server_handle.await;
     #[cfg(windows)]
     portmapper_handle.abort();