fix(rust): Window function had incorrect output name on ExprIR

[orlp]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 88.88889% with 1 line in your changes missing coverage. Please review.

Project coverage is 79.85%. Comparing base (a030634) to head (ee91baf).
Report is 24 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/polars-plan/src/plans/expr_ir.rs	88.88%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##             main   #18970    +/-   ##
========================================
  Coverage   79.85%   79.85%            
========================================
  Files        1523     1523            
  Lines      207315   207504   +189     
  Branches     2906     2904     -2     
========================================
+ Hits       165557   165709   +152     
- Misses      41210    41248    +38     
+ Partials      548      547     -1     

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