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

test/unit/agents/belief_collector/behaviours/test_continuous_collect.py

@@ -155,7 +155,7 @@ async def test_belief_text_values_not_lists(continuous_collector, mocker):
 async def test_belief_text_happy_path_logs_items_and_sends(continuous_collector, mocker):
     payload = {
         "type": "belief_extraction_text",
-        "beliefs": {"user_said": [["hello", "test"], ["No"]]}
+        "beliefs": {"user_said": ["hello test", "No"]}
     }
     # Your code calls self.send(..); patch it (or switch implementation to self.agent.send and patch that)
     continuous_collector.send = AsyncMock()