[WIP] Fix the failing GitHub Actions job test (ubuntu-latest, 3.10)

[Copilot]

• Fix KeyError: ('U', 336) in test_to_nc — scipy NetCDF writer can't handle Python list attrs
• Convert files attribute from list[str] to pipe-joined str in schema.py so to_netcdf() works with all backends
• Update all places in io.py that read attrs["files"] to split the string back to a list
• Verify test_to_nc passes