fix: clean up omnivoice import guard and __init__ error masking

Remove OmniVoice = None fallback in nodes/loader.py so missing omnivoice
gives a clear ImportError instead of a confusing AttributeError. Restore
__init__.py to clean form without the try/except that silently swallowed
real import errors. Add omnivoice mock to conftest.py and register a
pytest plugin that prevents pytest from treating the project root as a
Package node (which would try to import __init__.py outside a package
context and fail on the relative import).

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

nodes/loader.py

@@ -1,10 +1,7 @@
 import os
 import torch
 
-try:
-    from omnivoice import OmniVoice
-except ImportError:
-    OmniVoice = None  # type: ignore[assignment,misc]
+from omnivoice import OmniVoice
 
 try:
     import folder_paths