fix: guard omnivoice import in loader.py so node classes are importable without package

Wrap `from omnivoice import OmniVoice` in a try/except ImportError, setting
OmniVoice=None when absent. Add a clear runtime ImportError in load_model()
so users get an actionable message. Allows `from nodes.loader import
OmniVoiceModelLoader` to succeed outside of pytest (where conftest.py injects
the mock) while keeping all 13 tests green.

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

nodes/loader.py

@@ -1,7 +1,10 @@
 import os
 import torch
 
-from omnivoice import OmniVoice
+try:
+    from omnivoice import OmniVoice
+except ImportError:
+    OmniVoice = None  # deferred; will raise at runtime if package is missing
 
 try:
     import folder_paths
@@ -46,6 +49,11 @@ class OmniVoiceModelLoader:
     CATEGORY = "OmniVoice"
 
     def load_model(self, model_source, device, dtype, local_path=""):
+        if OmniVoice is None:
+            raise ImportError(
+                "omnivoice is not installed. Run: pip install omnivoice"
+            )
+
         if model_source == "Local path" and local_path:
             source = local_path
         else: