Design an in-memory TODO list API

### Object-Oriented Design: TODO List Design an in-memory TODO list component that supports the following operations: - `add(todo)` → returns an `id` - `delete(id)` → removes the entry if it exists - `get_todo(id)` → returns the TODO entry by id (or null/error if not found) - `get_all()` → returns all TODO entries You may assume there is a helper: - `check_todo(id) -> bool` which returns whether the entry has already been completed. **Clarify in your design:** - What fields a TODO has (e.g., title, description, created_at, completed flag) - Whether ordering matters in `get_all()` (insertion order? created time?) - Expected performance characteristics - How you handle missing ids and duplicates No UI is required; focus on clean API, data model, and internal data structures.