refactor(rust): Refactor coalesce_columns and add coalesce_binary_columns

[mansanlab]

The new code fixes a bug where the null check was always evaluating incorrectly, meaning the early exit never fired as intended. It also adds several shortcuts to skip expensive operations when a column is null or has no nulls and handles scalar broadcasting that the old code ignored. This replaces the sequential loop with a parallel tree reduction for multi-column inputs, resolving the TODO comment the original author left behind.

[codecov]

Codecov Report

❌ Patch coverage is 88.09524% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.88%. Comparing base (86f9210) to head (cd933ad).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/polars-ops/src/series/ops/horizontal.rs	88.09%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #27251      +/-   ##
==========================================
+ Coverage   81.56%   81.88%   +0.31%     
==========================================
  Files        1820     1820              
  Lines      251061   251093      +32     
  Branches     3149     3149              
==========================================
+ Hits       204788   205601     +813     
+ Misses      45465    44684     -781     
  Partials      808      808              

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