Complete UndoManager test coverage

[ZFordDev]

Closes #161

Expands UndoManager coverage to include:

• Undo delete restoring the original task and persistence call.
• Undo edit restoring the snapshot.
• Undo complete and uncomplete delegation.
• Empty-history behavior.
• MAX_HISTORY eviction order.
• Existing successful add and graceful failure behavior.

Validation:

• Ruff lint passes.
• All 8 UndoManager tests pass.
• The combined reminder, recurrence, and undo suites pass: 26 tests.