Handle unknown UID/GID in _OsBackend.stat() gracefully

[bzantium]

Summary

• _OsBackend.stat() calls pwd.getpwuid() and grp.getgrgid() to resolve file owner/group names, but raises KeyError when the UID/GID is not present in /etc/passwd or /etc/group.
• This is common in container environments and shared filesystems (e.g. NFS) where files may be owned by UIDs that don't exist locally.
• Falls back to the numeric UID/GID as a string, consistent with how ls -n and other standard tools handle unknown UIDs.

Fixes #790

Reproduction

# In a container where files are owned by UID 1000 but /etc/passwd has no entry for it:
from etils import epath
p = epath.Path("/some/file/owned/by/unknown/uid")
p.stat()  # KeyError: 'getpwuid(): uid not found: 1000'

Impact

This crashes downstream libraries like orbax-checkpoint which call epath.Path.stat() internally during checkpoint loading.

Test plan

• Verified fix resolves the crash in orbax-checkpoint on a shared filesystem with unknown UIDs
• Existing behavior unchanged for known UIDs