refactor(rust): Follow-up changes to polars-parquet

[coastalwhite]

This provides some small follow-up changes for #16747.

[codecov]

Codecov Report

Attention: Patch coverage is 69.76744% with 26 lines in your changes missing coverage. Please review.

Project coverage is 80.96%. Comparing base (d00ed0c) to head (f91cce7).

Files	Patch %	Lines
crates/polars-utils/src/fixedringbuffer.rs	31.42%	24 Missing ⚠️
...parquet/src/arrow/read/deserialize/nested_utils.rs	96.07%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16949      +/-   ##
==========================================
- Coverage   80.97%   80.96%   -0.02%     
==========================================
  Files        1445     1445              
  Lines      189825   189849      +24     
  Branches     2712     2712              
==========================================
- Hits       153717   153715       -2     
- Misses      35608    35634      +26     
  Partials      500      500              

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[codspeed-hq]

CodSpeed Performance Report

Merging #16949 will improve performances by 20.01%

_{Comparing coastalwhite:parquet-followup-changes (f91cce7) with main (d00ed0c)}

Summary

⚡ 1 improvements
✅ 36 untouched benchmarks

Benchmarks breakdown

	Benchmark	main	coastalwhite:parquet-followup-changes	Change
⚡	test_groupby_h2oai_q5	2.3 ms	1.9 ms	+20.01%