Relax/fix `test_concat_diagonal_invalid` error for `polars>=1.0.0`

[dangotbanned]

https://github.com/narwhals-dev/narwhals/actions/runs/21139459260/job/60789500820

FAILED tests/frame/concat_test.py::test_concat_diagonal_invalid[polars[lazy]] - narwhals.exceptions.NarwhalsError: type Datetime(Microseconds, None) is incompatible with expected type Int64
FAILED tests/frame/concat_test.py::test_concat_diagonal_invalid[polars[eager]] - narwhals.exceptions.NarwhalsError: type Datetime(Microseconds, None) is incompatible with expected type Int64

I thought I'd caught all of these in (ceceb5b), (75d5eac) and (f5a6fdc).
Seems like the error changed some time after 1.0.0, not sure when though yet