fix: beliefs now adhere to expected format

[
    -before user_said belief was a list of lists of strings, now it's a list of strings
]

[ref]: N25B-206

src/control_backend/agents/mock_agents/belief_text_mock.py

@@ -12,7 +12,7 @@ class BeliefTextAgent(Agent):
             # Send multiple beliefs in one JSON payload
             payload = {
                 "type": "belief_extraction_text",
-                "beliefs": {"user_said": [["hello test"],["Can you help me?"],["stop talking to me"],["No"],["Pepper do a dance"]]}     
+                "beliefs": {"user_said": ["hello test","Can you help me?","stop talking to me","No","Pepper do a dance"]}     
             }
 
             msg = Message(to=to_jid)