Fix slice fill behavior when iterable divides evenly

[BlocksecPHD]

Summary

• avoid appending fill_with when the iterable length divides evenly by the requested slice count
• add sync and async regression tests for the even-division case

Testing

• PYTHONPATH=src ./.venv/bin/pytest -q tests/test_filters.py -k 'slice'\n- PYTHONPATH=src ./.venv/bin/pytest -q tests/test_async_filters.py -k 'slice'\n- ./.venv/bin/uv run --group tests pytest -q tests/test_filters.py -k 'slice'\n- ./.venv/bin/uv run --group tests pytest -q tests/test_async_filters.py -k 'slice'