fix: Expression pl.concat was incorrectly marked as elementwise

[orlp]

Fixes #21980.
Fixes #19877.

Also fixes an issue that wasn't reported (to my knowledge) with pl.concat in group_by.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.90%. Comparing base (90094f0) to head (9243bd4).
Report is 8 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #22019      +/-   ##
==========================================
- Coverage   80.99%   80.90%   -0.09%     
==========================================
  Files        1638     1639       +1     
  Lines      236856   236863       +7     
  Branches     2696     2696              
==========================================
- Hits       191840   191643     -197     
- Misses      44382    44586     +204     
  Partials      634      634              

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