fix: Fix decimal arithmetic schema

[ritchie46]

fixes #20367

[codecov]

Codecov Report

Attention: Patch coverage is 90.62500% with 3 lines in your changes missing coverage. Please review.

Project coverage is 78.96%. Comparing base (dbd2c5f) to head (a95f184).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/polars-plan/src/plans/aexpr/schema.rs	83.33%	3 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #20398   +/-   ##
=======================================
  Coverage   78.95%   78.96%           
=======================================
  Files        1561     1561           
  Lines      219881   219908   +27     
  Branches     2471     2471           
=======================================
+ Hits       173614   173642   +28     
+ Misses      45699    45698    -1     
  Partials      568      568           

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