fix: Fix error filtering after with_columns() on unit height LazyFrame

[nameexhaustion]

Fixes #20489

Replaces all uses of projects_as_scalar() with is_scalar() to be consistent with the strictness check used by with_columns()

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.98%. Comparing base (f104170) to head (a75a9d6).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #20584      +/-   ##
==========================================
- Coverage   78.98%   78.98%   -0.01%     
==========================================
  Files        1564     1564              
  Lines      220765   220759       -6     
  Branches     2504     2504              
==========================================
- Hits       174368   174361       -7     
- Misses      45823    45824       +1     
  Partials      574      574              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.