fix: Merge Parquet nested and flat decoders

[coastalwhite]

Fixes #21332.

[codecov]

Codecov Report

Attention: Patch coverage is 77.98165% with 48 lines in your changes missing coverage. Please review.

Project coverage is 79.93%. Comparing base (a3072d2) to head (b2f2d05).
Report is 12 commits behind head on main.

Files with missing lines	Patch %	Lines
...olars-parquet/src/arrow/read/deserialize/simple.rs	66.43%	48 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #21342      +/-   ##
==========================================
+ Coverage   79.92%   79.93%   +0.01%     
==========================================
  Files        1596     1596              
  Lines      228642   228619      -23     
  Branches     2615     2616       +1     
==========================================
+ Hits       182733   182753      +20     
+ Misses      45311    45267      -44     
- Partials      598      599       +1     

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