fix: skip vanished entries when listing wal dirs

[ethe]

Summary

• Treat NotFound from DirEntry::metadata as a benign race so listings continue when files disappear between read_dir and metadata.
• Apply the behavior uniformly across tokio, tokio-uring, and monoio disk backends for consistent runtime semantics.
• Add a regression test (tokio backend) to ensure disappearing entries are skipped and do not turn into hard errors.

Details

• Introduced helper in tokio backend to filter vanished entries while preserving other I/O errors.
• Monoio and tokio-uring now mirror the same NotFound skip.
• Regression test exercises the race by deleting a file after read_dir and asserting only existing entries are returned.