test: add descending cursor direction pagination coverage

[kianjones9]

Summary

• Add focused unit tests for pagination cursor direction logic
• Covers asc forward, desc forward, backward traversal, and default-order behavior

Notes

• This PR is intentionally stacked on top of fix: make auto-pagination cursor direction order-aware #78
• Merge this after fix: make auto-pagination cursor direction order-aware #78, or retarget base to main once fix: make auto-pagination cursor direction order-aware #78 lands

Test run

• uv run --with pytest --with pytest-asyncio --with pytest-xdist pytest tests/test_pagination_cursor_direction.py -q
• Result: 4 passed