fix(rust): Add missing chunk align in pipe sink

[orlp]

Fixes a hung test test_streaming_group_by_struct_key for me.

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 79.84%. Comparing base (b2550a0) to head (affc490).
Report is 38 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/polars-pipe/src/executors/sinks/io.rs	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #18457      +/-   ##
==========================================
- Coverage   79.86%   79.84%   -0.02%     
==========================================
  Files        1496     1497       +1     
  Lines      200364   201828    +1464     
  Branches     2867     2867              
==========================================
+ Hits       160012   161158    +1146     
- Misses      39806    40124     +318     
  Partials      546      546              

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