Handle Windows PermissionDenied during storage footprint sync sweep

[Copilot]

The Test Suite (windows-latest, stable) job failed in paper_bench::storage_footprint because benchmark runs errored with Os { code: 5, kind: PermissionDenied, message: "Access is denied." }. The failure came from treating all sync_all() errors as fatal while traversing benchmark store files on Windows.

• Root cause

  • sync_tree() unconditionally propagated file.sync_all() errors, including Windows-specific permission denials on files created/managed by the benchmarked storage engines.

• Targeted change

  • Updated src/paper_bench/storage_footprint.rs so sync_tree() ignores std::io::ErrorKind::PermissionDenied for file sync_all() calls.
  • Preserved existing behavior for all other error kinds (still fail fast).

• Behavioral impact

  • Benchmark storage footprint tests no longer fail due to non-actionable Windows permission denials during fsync traversal.
  • Real I/O failures remain visible and continue to abort the run.

let file = File::open(&path)?;
if let Err(err) = file.sync_all() {
    if err.kind() != std::io::ErrorKind::PermissionDenied {
        return Err(err.into());
    }
}