fix: Ensure aligned chunks in OOC sort

[ritchie46]

fixes #19111

[codecov]

Codecov Report

Attention: Patch coverage is 94.44444% with 2 lines in your changes missing coverage. Please review.

Project coverage is 79.79%. Comparing base (c4a28ae) to head (24be32e).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/polars-core/src/frame/mod.rs	96.00%	1 Missing ⚠️
crates/polars-pipe/src/executors/sinks/io.rs	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #19118      +/-   ##
==========================================
- Coverage   79.79%   79.79%   -0.01%     
==========================================
  Files        1531     1531              
  Lines      208556   208573      +17     
  Branches     2913     2913              
==========================================
+ Hits       166423   166436      +13     
- Misses      41582    41586       +4     
  Partials      551      551              

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