fix: Support duplicate names in over

[kdn36]

Fixes #27443

• Support duplicate names in over, undo fix: Raise for duplicate columns in over() #26968
• Revert to the numeric fast path from fix: Fix unary expressions on literal in over context #26827
• Fix the newly found issue on non-numeric aggregations, for MRE see Regression some time after 1.38.1 when using multiple expression in .over with same alias #27443
• Fixup original_len internal code incorrectness issues (no github issue)

[codecov]

Codecov Report

❌ Patch coverage is 92.59259% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.15%. Comparing base (7fe21f6) to head (f3b4a77).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/polars-expr/src/expressions/window.rs	87.50%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #27544      +/-   ##
==========================================
- Coverage   81.43%   81.15%   -0.28%     
==========================================
  Files        1837     1837              
  Lines      255164   255165       +1     
  Branches     3179     3179              
==========================================
- Hits       207782   207076     -706     
- Misses      46558    47265     +707     
  Partials      824      824              

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