fix(ragfs): skip vanished localfs entries during listing

[yu1128-L]

Fixes #5580.

What changed

localfs currently aborts the entire directory listing if an entry returned by readdir disappears before its metadata is read. This is a normal race for runtime lock files and makes otherwise healthy listings fail with NotFound.

This change treats only ErrorKind::NotFound from DirEntry::metadata() as a vanished entry and skips it. Other metadata errors are still propagated.

Tests

• cargo test -p ragfs test_listing_skips_entry_removed_after_readdir -- --nocapture
• The targeted regression test passes.
• Existing localfs coverage was also exercised; the unrelated permission-based glob test cannot produce chmod 0 failures when run as Docker root.