Handle Edge Case in _last_pos_filled Calculation

[Susmita331]

Brief description of what is fixed or changed

• Fixed an edge case where _last_pos_filled could incorrectly stop at the last occupied index and miss earlier valid elements.
• Previously, _last_pos_filled was determined using i = -1 and scanning backward, which failed in cases where the last valid element wasn’t at the highest index.
• The function now starts from self._size - 1 and scans properly, ensuring that all valid elements are preserved before shrinking the array.

Other comments

• Tested with multiple scenarios, including cases where valid elements were scattered across _data.
• Future improvements: Optimize _modify() to avoid unnecessary iterations and improve performance.
• Maintainers' feedback is welcome if further refinements are needed.