Support general iterables in natural_list

[Pxnzerr]

Summary

natural_list() documented its argument as items (Iterable): An iterable of items, but:

1. The type annotation was restricted to items: list[Any].
2. The implementation relied on sequence indexing (items[0], items[:-1], items[-1]) and len(items), raising TypeError when passed generators, sets, dict_keys, iterators, or custom non-subscriptable iterables.

Changes

• Updated natural_list type annotation to items: Iterable[Any] (importing Iterable in TYPE_CHECKING).
• Normalized items using list comprehension (item_list = [str(item) for item in items]), safely handling any finite iterable in a single pass.
• Added comprehensive unit test coverage in tests/test_lists.py for tuples, sets, generators, dict_keys, and range.

Validation

• pytest tests/test_lists.py: 14 passed
• pytest: 706 passed
• mypy src tests/test_lists.py: clean
• ruff check .: clean

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.69%. Comparing base (bada0f8) to head (da44e1c).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #381   +/-   ##
=======================================
  Coverage   99.69%   99.69%           
=======================================
  Files          12       12           
  Lines         980      983    +3     
=======================================
+ Hits          977      980    +3     
  Misses          3        3           

Flag	Coverage Δ
macos-latest	97.55% <100.00%> (+<0.01%)	⬆️
ubuntu-latest	97.55% <100.00%> (+<0.01%)	⬆️
windows-latest	93.18% <100.00%> (+0.02%)	⬆️

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