Enhance MLX Audio Documentation and Testing Framework

- Updated MLX_AUDIO.md to reflect validated status and included detailed validation results, model mapping, and API usage examples.
- Added a demo script (demo.py) for testing audio generation speed and functionality.
- Introduced a test script (test_tts.py) to validate MLX audio model loading and generation, ensuring robust testing for future developments.
- Created a .gitignore file in the mlx-test directory to exclude unnecessary files from version control.

mlx-test/.gitignore

@@ -0,0 +1,9 @@
+# Virtual environment
+venv/
+
+# Generated test files
+*.wav
+
+# Python cache
+__pycache__/
+*.pyc