feat: add OmniVoiceModelLoader node

Implements OmniVoiceModelLoader with INPUT_TYPES, RETURN_TYPES, and
load_model supporting both HuggingFace auto-download and local path
sources. Adds TDD test suite and pytest infrastructure (conftest.py,
pytest.ini) to enable testing outside ComfyUI without omnivoice installed.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

conftest.py

@@ -0,0 +1,6 @@
+import sys
+import os
+
+# Ensure the project root is on sys.path so `nodes.loader` can be imported
+# as a top-level package without needing to install the package.
+sys.path.insert(0, os.path.dirname(__file__))