[Week7 - hjy] Task4: Add comprehensive pagination and sorting tests

[HyungjoonYang]

Summary

• Added 26 new test functions across test_notes.py and test_action_items.py
• No production code changes — test-only task

Test plan

Notes tests (13 new):

• Pagination: limit, skip, skip+limit, skip beyond total, page overlap, empty DB
• Sorting: title asc/desc, created_at asc/desc, invalid field fallback
• Search + pagination combo

Action items tests (13 new):

• Pagination: limit, skip, skip+limit, skip beyond total, page overlap, empty DB
• Sorting: description asc/desc, created_at asc/desc, invalid field fallback
• Filter: completed=true with pagination, completed=false, no matches → empty
• All 29 tests pass

Notable tradeoffs

• Uses time.sleep(0.01) between creates for deterministic created_at ordering (SQLite datetime resolution)
• Page overlap test verifies no duplicate IDs across pages (catches off-by-one in skip/limit)
• Invalid sort field tests current fallback behavior rather than enforcing 400

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