fix(python, rust): Block rounding/truncating to negative durations

[rob-sil]

Polars will currently round dates and datetimes to negative durations:

In [16]: pl.Series([datetime(2020, 2, 15)]).dt.round('-1mo')
Out[16]:
shape: (1,)
Series: '' [datetime[μs]]
[
        2020-02-01 00:00:00
]

(Except not during development where negative durations hit a debug_assert.)

This PR disables that behavior for both rounding and truncating to negative durations, as discussed in #12597

[MarcoGorelli]

Nice one, thanks @rob-sil !

[MarcoGorelli]

Merging then, as there's been no objections - let's take things forwards in the other PR!