fix: Properly update groups after gather in aggregation context

[JakubValtar]

I traced #21178 down to gather in aggregation context not updating the groups properly.

This PR makes sure the groups are updated, otherwise this causes wrong results or OOB access in the following operation. I checked that each changed line fixes the matching test.

I added Python tests to cover each of the changed lines, but they are a bit fragile, since there is no guarantee they will keep triggering this Rust code in the future. Is there a better way I can write the tests, maybe as Rust unit tests?

Closes #21178

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.90%. Comparing base (fb28c07) to head (a5d1c92).
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #21369      +/-   ##
==========================================
- Coverage   79.91%   79.90%   -0.01%     
==========================================
  Files        1597     1597              
  Lines      228713   228715       +2     
  Branches     2618     2618              
==========================================
- Hits       182767   182760       -7     
- Misses      45347    45356       +9     
  Partials      599      599              

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[ritchie46]

That's a nasty one.

I added Python tests to cover each of the changed lines, but they are a bit fragile, since there is no guarantee they will keep triggering this Rust code in the future. Is there a better way I can write the tests, maybe as Rust unit tests?

The preferred tests are Python tests, but if you are not sure you can hit it (or think it will change), a unit test in the crate is also fine.

I will merge this regardless. If we do a release this afternoon, it's great to have this included. Thanks a lot!