Summary

• Fixes the slice filter incorrectly appending fill_with to every slice when the iterable length is evenly divisible by the slice count.
• Root cause: When length % slices == 0, slices_with_extra is 0, making the condition slice_number >= slices_with_extra always True. This causes fill_with to be appended to every slice even though no padding is needed.
• Fix: Add a truthiness check for slices_with_extra before the comparison, so fill_with is only appended when there are actually shorter slices that need padding.

Before (broken):

>>> [1,2,3,4]|slice(4, 'foo')|list
[[1, 'foo'], [2, 'foo'], [3, 'foo'], [4, 'foo']]

After (fixed):

>>> [1,2,3,4]|slice(4, 'foo')|list
[[1], [2], [3], [4]]

Fixes #2118

Test plan

• Added test_slice_evenly_divisible covering evenly divisible cases (4 items / 4 slices, 6 items / 2 slices)
• Verified existing test_slice still passes (non-evenly-divisible case with fill_with works correctly)
• All 133 sync filter tests and 2 async slice tests pass