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>

__init__.py

@@ -1,18 +1,13 @@
-try:
-    from .nodes import OmniVoiceModelLoader, OmniVoiceGenerate
+from .nodes import OmniVoiceModelLoader, OmniVoiceGenerate
 
-    NODE_CLASS_MAPPINGS = {
-        "OmniVoiceModelLoader": OmniVoiceModelLoader,
-        "OmniVoiceGenerate": OmniVoiceGenerate,
-    }
+NODE_CLASS_MAPPINGS = {
+    "OmniVoiceModelLoader": OmniVoiceModelLoader,
+    "OmniVoiceGenerate": OmniVoiceGenerate,
+}
 
-    NODE_DISPLAY_NAME_MAPPINGS = {
-        "OmniVoiceModelLoader": "OmniVoice Model Loader",
-        "OmniVoiceGenerate": "OmniVoice Generate",
-    }
-except ImportError:
-    # Graceful fallback when loaded outside of a package context (e.g. pytest)
-    NODE_CLASS_MAPPINGS = {}
-    NODE_DISPLAY_NAME_MAPPINGS = {}
+NODE_DISPLAY_NAME_MAPPINGS = {
+    "OmniVoiceModelLoader": "OmniVoice Model Loader",
+    "OmniVoiceGenerate": "OmniVoice Generate",
+}
 
 __all__ = ["NODE_CLASS_MAPPINGS", "NODE_DISPLAY_NAME_MAPPINGS"]