Unify map() list-row handling across list representations

[kosiew]

Which issue does this PR close?

• Part of Unify list-like row decomposition for map construction #22986

Rationale for this change

map() had separate decomposition and null-row filtering paths for List/LargeList and FixedSizeList, resulting in duplicated logic and making it harder to verify equivalent behavior across supported outer list representations.

This change introduces shared cross-representation tests and consolidates the internal row access and decomposition logic behind a single private adapter while preserving existing behavior and error handling.

What changes are included in this PR?

• Introduce a private ListArrayRepresentation adapter to provide a common row-oriented interface for List, LargeList, and FixedSizeList.

• Route map decomposition through the shared adapter instead of maintaining separate constructor paths for variable-sized and fixed-size lists.

• Reuse the same key-driven null-row filtering logic for both key and value rows.

• Route key validation through the shared row adapter.

• Remove duplicated helper functions and the dedicated fixed-size list construction path that are superseded by the unified implementation.

• Add reusable test fixtures and assertions for validating complete MapArray outputs.

• Add cross-representation tests verifying equivalent behavior for:

  • null rows interleaved with non-null rows,
  • sliced inputs containing null rows,
  • all-null outer rows.

• Verify map offsets, outer validity bitmap, and child key/value arrays are identical across supported list representations.

Are these changes tested?

Yes.

The following tests were added:

• test_make_map_null_rows_are_equivalent_across_list_representations
• test_make_map_sliced_null_rows_are_equivalent_across_list_representations
• test_make_map_all_null_rows_are_equivalent_across_list_representations

These complement the existing map tests by asserting equivalent behavior for List, LargeList, and FixedSizeList representations without changing expected outputs.

Are there any user-facing changes?

No. This is an internal refactoring and test expansion intended to preserve existing behavior and error messages while reducing duplicated implementation code.

LLM-generated code disclosure

This PR includes LLM-generated code and comments. All LLM-generated content has been manually reviewed.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 97.19101% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.71%. Comparing base (18b1e35) to head (1f84fcb).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
datafusion/functions-nested/src/map.rs	97.19%	4 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##             main   #23859    +/-   ##
========================================
  Coverage   80.70%   80.71%            
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  Files        1090     1090            
  Lines      370150   370371   +221     
  Branches   370150   370371   +221     
========================================
+ Hits       298746   298960   +214     
+ Misses      53598    53596     -2     
- Partials    17806    17815     +9     

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