Add unit tests for service functionality and model caching

- Implement tests for ben2 and da3 model caching to ensure models are reused for matching runtime keys.
- Verify service state management, including client registration, job cancellation, and client expiration.
- Test discovery file creation and removal based on service ownership.
- Ensure frame extraction functionality works without models and produces expected results.

src/snapform/image_mesh.py

@@ -517,13 +517,13 @@ class ConvertImageToMesh(bpy.types.Operator):
     def poll(cls, context):
         from .models import MODELS
         from .operators import active_job
-        from .runtime import host_busy, model_ready
+        from .client import service_busy, model_ready
 
         return (
             is_image_empty(context.object)
             and video_total_frames(context.object.data) == 1
             and active_job() is None
-            and not host_busy()
+            and not service_busy()
             and any(model_ready(model_key) for model_key in MODELS)
         )