Text Document with Undo/Redo

Implement a TextDocument class that supports appending text, deleting text from the end, and undo/redo of those operations.

Operations:

• TextDocument() — initialise an empty document.
• append(text) — append the string text to the end of the document. Returns None.
• deleteFromEnd(numChars) — delete the last numChars characters. If the document has fewer than numChars characters, delete everything. A deletion that removed zero characters (empty document) is a no-op and must not be added to the undo stack. Returns None.
• undo() — undo the most recent append or non-empty deleteFromEnd. If there is nothing to undo, no-op. Returns None.
• redo() — redo the most recently undone operation. Any new append or deleteFromEnd clears the redo stack. Returns None.
• getContent() — return the current document contents (a string).

Class name: TextDocument (first operation in test_cases).