KeyError in _OsBackend.stat() for unknown UID/GID in container environments

[bzantium]

Bug

_OsBackend.stat() in etils/epath/backend.py crashes with KeyError when a file is owned by a UID/GID not present in /etc/passwd or /etc/group.

This is common in:

• Container environments where the host UID doesn't exist inside the container
• Shared/network filesystems (NFS, FUSE) where files are owned by remote UIDs

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'

Traceback

File "etils/epath/gpath.py", line 293, in stat
    return self._backend.stat(self._path_str)
File "etils/epath/backend.py", line 235, in stat
    owner = pwd.getpwuid(st.st_uid).pw_name
KeyError: 'getpwuid(): uid not found: 1000'

Impact

Downstream libraries like orbax-checkpoint call epath.Path.stat() during checkpoint loading, causing crashes that are difficult to work around.

Suggested fix

Catch KeyError and fall back to the numeric UID/GID as a string, consistent with how ls -n handles unknown UIDs.

See PR #789.