fix: handle string keys in hotkey and hold_and_press

[shotsan]

Fixes #195

Code Issues

• Target Methods: The hotkey() and hold_and_press() methods located in WindowsOSACI.py, MacOSACI.py, and LinuxOSACI.py.
• The Problem: These methods expected the keys, hold_keys, and press_keys arguments to strictly be lists (e.g., ['enter']), but failed to enforce this or handle edge cases where a single string (e.g., 'enter') was passed by the agent.

Details of Logic Errors

• String Iteration Bug: In Python, strings are iterable objects. When a raw string like 'enter' was passed to the _normalize_key(k) for k in keys list comprehension, the method iterated over the string character-by-character.
• The Result: Instead of generating a pyautogui script that strikes the Enter key, it incorrectly generated a sequence to press the individual letters: e, n, t, e, r.

The Fix

• Added an isinstance(keys, str) check at the beginning of these methods.
• If a string is detected, it is wrapped in a list (keys = [keys]). This forces the list comprehension to iterate over the entire string as a single item, resolving the bug while maintaining backward compatibility for list inputs.

Tests Added

• Added tests/test_aci_hotkey.py to ensure strings like 'enter' and lists like ['alt', 'tab'] both compile the correct pyautogui commands across all OS implementations.