perf: Improve streaming groupby CSE

[orlp]

This ensures duplicate aggregates (not just inputs to the aggregates) are only computed once.

[codecov]

Codecov Report

Attention: Patch coverage is 94.44444% with 1 line in your changes missing coverage. Please review.

Project coverage is 80.22%. Comparing base (665a202) to head (582dd88).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
.../polars-stream/src/physical_plan/lower_group_by.rs	94.44%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #23092      +/-   ##
==========================================
+ Coverage   80.20%   80.22%   +0.02%     
==========================================
  Files        1684     1684              
  Lines      223918   223936      +18     
  Branches     2809     2809              
==========================================
+ Hits       179586   179653      +67     
+ Misses      43672    43623      -49     
  Partials      660      660              

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