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.

tests/test_pack.py

@@ -61,7 +61,7 @@ class PackTest(unittest.TestCase):
                     names,
                 )
                 self.assertIn("worker.py", names)
-                self.assertIn("ai_host.py", names)
+                self.assertIn("service.py", names)
                 self.assertNotIn("snapform/__init__.py", names)
                 self.assertNotIn("importer.py", names)
                 self.assertIn("vendor/depth-anything-3.tar.gz", names)