fix: bound detection QA to inference coverage

backend/tests/test_sprint8c_detection_visualization_qa.py

@@ -42,6 +42,9 @@ class FakeQuery:
     def first(self):
         return self.rows[0] if self.rows else None
 
+    def count(self):
+        return len(self.rows)
+
 
 class FakeSession:
     def __init__(self, objects=None, query_rows=None) -> None: