fix: Ensure Function name correctness in cse

[ritchie46]

fixes #20927

[github-actions]

The uncompressed lib size after this PR is 38.3617 MB.

[github-actions]

The uncompressed lib size after this PR is 38.3617 MB.

[codecov]

Codecov Report

Attention: Patch coverage is 73.33333% with 8 lines in your changes missing coverage. Please review.

Project coverage is 79.30%. Comparing base (56fb8c4) to head (2ee577b).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
...es/polars-plan/src/plans/optimizer/cse/cse_expr.rs	75.86%	7 Missing ⚠️
crates/polars-plan/src/utils.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #20929      +/-   ##
==========================================
- Coverage   79.76%   79.30%   -0.46%     
==========================================
  Files        1569     1578       +9     
  Lines      222730   224135    +1405     
  Branches     2574     2574              
==========================================
+ Hits       177651   177749      +98     
- Misses      44488    45795    +1307     
  Partials      591      591              

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