fix: `rolling_*_by was throwing incorrect error when dataframe was sorted by contained multiple chunks

[MarcoGorelli]

closes #16246

[MarcoGorelli]

The error comes from L112

    let by_values = by.cont_slice().map_err(|_| {
        polars_err!(
            ComputeError:
            "`by` column should not have null values in 'rolling by' expression"
        )
    })?;

The values already get re-chunked, so the by ones probably should too?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.86%. Comparing base (5b77f01) to head (8aa2d87).
Report is 18 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16247      +/-   ##
==========================================
- Coverage   81.04%   80.86%   -0.18%     
==========================================
  Files        1392     1394       +2     
  Lines      178931   179489     +558     
  Branches     2907     2913       +6     
==========================================
+ Hits       145014   145145     +131     
- Misses      33411    33841     +430     
+ Partials      506      503       -3     

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