fix: Properly handle to_physical_repr of nested types

[coastalwhite]

Fixes #20308.

[coastalwhite]

This is a drive-by change. This name is much better for this function.

[codecov]

Codecov Report

Attention: Patch coverage is 96.73203% with 5 lines in your changes missing coverage. Please review.

Project coverage is 79.01%. Comparing base (62ebbe5) to head (4f7bc98).
Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/polars-expr/src/groups/row_encoded.rs	0.00%	4 Missing ⚠️
...rates/polars-core/src/chunked_array/struct_/mod.rs	97.95%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #20413      +/-   ##
==========================================
+ Coverage   78.97%   79.01%   +0.03%     
==========================================
  Files        1562     1562              
  Lines      220103   220105       +2     
  Branches     2486     2486              
==========================================
+ Hits       173821   173910      +89     
+ Misses      45709    45622      -87     
  Partials      573      573              

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