Problem: Text Editor with Undo/Redo

Design and implement a text document editor that supports the following operations:

• insert(chars): Appends the given characters to the end of the document.
• delete(k): Removes up to k characters from the end of the document. If k exceeds the current document length, all existing characters are removed.
• undo_last(): Undoes the most recent effective operation.
• redo_last(): Re-applies the last undone operation.

The design must include an Operation interface (or abstract class) with two methods:

• apply(doc) -> doc: Applies the operation to a given document state and returns the new document state.
• rollback_operation() -> Operation: Returns an inverse Operation object that can undo the effects of the current operation.

The editor must maintain separate undo and redo stacks. A crucial constraint is that applying any new insert or delete operation must clear the entire redo stack.

A get_current_content() -> str method must be provided to retrieve the current document content.

The implementation should robustly handle various edge cases, including:

• Consecutive delete operations.
• Attempting to delete from an empty document.
• Calling undo_last() or redo_last() when their respective stacks are empty.
• Ensuring that only effective delete operations (i.e., those that actually remove characters) are recorded for undo.

Candidates are required to analyze the time and space complexity for each operation and justify their data structure choices.

Unit tests should be provided to cover the following scenarios:

• Simple undo/redo flows.
• Multiple consecutive delete operations.
• delete operations where k is very large and exceeds the current document length.
• Sequences of operations that contain no insert operations.
• Idempotent redo_last() calls when no operations are available to be redone.