fix(rust): Incorrect arg_sort with descending+limit

[boris324]

Summary

• Fix arg_sort and arg_sort_no_nulls to use the correct comparator when descending=true and a limit is set
• select_nth_unstable_by was always using ascending comparison to partition elements, causing the smallest N elements to be selected instead of the largest N when descending=true
• Add tests for arg_sort with descending+limit, ascending+limit, and descending+limit+nulls

Closes #26833

Test plan

• Added test_arg_sort_descending_with_limit - verifies top-3 largest indices are returned
• Added test_arg_sort_asc_with_limit - verifies top-3 smallest indices (regression guard)
• Added test_arg_sort_desc_limit_nulls - verifies descending+limit with null values

[codecov]

Codecov Report

❌ Patch coverage is 96.07843% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.68%. Comparing base (9d60226) to head (d3601c4).
⚠️ Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
...polars-core/src/chunked_array/ops/sort/arg_sort.rs	96.07%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #26839      +/-   ##
==========================================
+ Coverage   81.00%   81.68%   +0.68%     
==========================================
  Files        1805     1805              
  Lines      248021   248066      +45     
  Branches     3132     3132              
==========================================
+ Hits       200902   202635    +1733     
+ Misses      46313    44625    -1688     
  Partials      806      806              

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