fix: Raise error instead of panic for unsupported pivot aggregate

[RenzoMXD]

The panic was caused by .expect() in IRBuilder::group_by() (builder_ir.rs:303) when schema derivation failed for an unsupported aggregate expression. Changed group_by() to return PolarsResult<Self> so the error propagates as an InvalidOperationError instead of panicking.

Closes #25860
Closes #26843

[codecov]

Codecov Report

❌ Patch coverage is 80.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 81.65%. Comparing base (9d60226) to head (d34c594).

Files with missing lines	Patch %	Lines
...rc/plans/optimizer/projection_pushdown/group_by.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #26863      +/-   ##
==========================================
+ Coverage   81.00%   81.65%   +0.65%     
==========================================
  Files        1805     1805              
  Lines      248021   248020       -1     
  Branches     3132     3132              
==========================================
+ Hits       200902   202530    +1628     
+ Misses      46313    44684    -1629     
  Partials      806      806              

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