🐛 fix(flow.py): change flow field type from str to Dict to allow for JSON data

✨ feat(flow.py): add validator to ensure flow field is a valid JSON object with required fields
The flow field in the FlowBase model has been changed from a string to a dictionary to allow for JSON data. A validator has been added to ensure that the flow field is a valid JSON object with the required fields. The tests have been updated to reflect these changes.

tests/conftest.py

@@ -108,6 +108,5 @@ def client_fixture(session: Session):  #
 
     app.dependency_overrides[get_session] = get_session_override  #
 
-    client = TestClient(app)  #
-    yield client  #
+    yield TestClient(app)
     app.dependency_overrides.clear()  #