perf: Split last rowgroup among all threads in new-streaming parquet reader

[orlp]

This isn't very flexible but at least ensures that for single-rowgroup files we're parallel even if it would be smaller than the ideal row group size, in case it feeds into an expensive operator.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 30 lines in your changes missing coverage. Please review.

Project coverage is 79.16%. Comparing base (8837dc8) to head (be60857).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
...polars-stream/src/nodes/io_sources/parquet/init.rs	0.00%	30 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #21027      +/-   ##
==========================================
+ Coverage   79.14%   79.16%   +0.02%     
==========================================
  Files        1583     1583              
  Lines      225119   225248     +129     
  Branches     2581     2586       +5     
==========================================
+ Hits       178162   178324     +162     
+ Misses      46367    46334      -33     
  Partials      590      590              

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