fix(rust): Replace panic with error when sorting object dtype columns

[Kevin-Patyk]

Resolves #24677.

The introduced fix places a fast fail after the empty checks in sort_impl(...) and arg_sort(...). It checks if the column you want to sort by (sort key) is an Object data type using DataType::is_object(). If it is, it bails.

As a note, a test was updated in test_assert_frame_equal.py since it needed to match the exception.

Claude Sonnet 4.5 was used to help me navigate the codebase.

[codecov]

Codecov Report

❌ Patch coverage is 80.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.39%. Comparing base (4929540) to head (fec85ff).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
...ates/polars-core/src/chunked_array/ops/sort/mod.rs	60.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #26601      +/-   ##
==========================================
+ Coverage   81.37%   81.39%   +0.01%     
==========================================
  Files        1794     1794              
  Lines      244998   245008      +10     
  Branches     3079     3079              
==========================================
+ Hits       199379   199415      +36     
+ Misses      44833    44807      -26     
  Partials      786      786              

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