Adds comprehensive unit tests for VoiceTTS class:
- __init__ with pyttsx3 import failure (mocked)
- speak() when not available (skips gracefully)
- set_rate() and set_volume() property updates
- get_voices() when engine is None
- set_voice() with/without engine
- available property behavior
All tests mock pyttsx3 so they run without audio hardware.
Fixes#768
