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/models.py

@@ -83,7 +83,7 @@ _ENUM_ITEMS_CACHE = {}
 def enum_items(supports_gs=None, show_download_status=False):
     model_ready = None
     if show_download_status:
-        from .runtime import model_ready
+        from .client import model_ready
 
     models = [
         model