Preserve trailing-comma layout when sorting reexports

[PSR94]

Summary

Preserve explicitly multiline __all__ collections with trailing commas when --sort-reexports runs under trailing-comma-aware formatting such as the Black profile.

Why this helps

This keeps sorted reexports stable with Black's magic trailing comma behavior, so short __all__ declarations that are intentionally one item per line are not collapsed after sorting.

Changes made

• Detect when a source literal is multiline and ends with a trailing comma.
• Preserve that multiline layout when the active config includes trailing commas.
• Add regression coverage for short tuple-style and list-style __all__ exports.

Testing

• uv run pytest tests/unit/test_regressions.py -k '2578 or 2280'
• uv run pytest tests/unit/test_isort.py -k 'reexport_multiline'
• uv run ruff format --check isort/literal.py tests/unit/test_regressions.py
• uv run ruff check isort/literal.py tests/unit/test_regressions.py
• uv run mypy isort tests
• ./scripts/done.sh

Closes #2578

[codecov]

Codecov Report

❌ Patch coverage is 92.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 99.36%. Comparing base (0145f77) to head (4fda57b).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2605      +/-   ##
==========================================
- Coverage   99.43%   99.36%   -0.07%     
==========================================
  Files          41       41              
  Lines        3159     3168       +9     
  Branches      685      687       +2     
==========================================
+ Hits         3141     3148       +7     
- Misses         11       12       +1     
- Partials        7        8       +1     

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[DanielNoord]

Do you think you can get full coverage on this? I have not looked at the code yet but we prefer all new code to be covered by tests :)

[PSR94]

Thanks for the review. I updated the PR to add coverage for the new literal formatting paths, including the formatting-function branch and the trailing-comma detection guard. I also reran uv run ruff format --check isort/literal.py tests/unit/test_literal.py tests/unit/test_regressions.py, uv run ruff check isort/literal.py tests/unit/test_literal.py tests/unit/test_regressions.py, uv run mypy isort tests, and ./scripts/done.sh; they are passing, and isort/literal.py reports 100% coverage locally.