Allow Item Assignment with __setitem__

Implement the __setitem__ method for the ExpandableList class so that it behaves like a list for indexing and assignment but with the following extended behavior: - If an assignment uses an index within the current bounds, it replaces the existing element (like a normal list). - If an assignment uses a positive index that is equal to or greater than the current length, the list should be automatically extended with None values to reach that index and then the value should be placed there. Example: assigning to index 3 in a list of length 1 should extend the list to length 4. - Negative indices should behave like Python lists (e.g., -1 refers to the last element). If a negative index is out of range, raise IndexError (the same as a list would). You will complete the TODO in __setitem__. A helper function assign_and_get(initial, index, value) is provided and used in tests.

Write a class that supports indexing and assignment. When assigning past the end, the container should expand filling gaps with None.

- Do not use built-in list methods that trivially implement the extension behavior for you (you may use append, extend, etc., but the logic must follow the rules). - Maintain the semantics of negative indices as Python lists. - Raise IndexError for out-of-range negative indices as a normal list would.