feat(api): Enhance persistence logic and refactor codebase structure

tests/api/test_agent.py

@@ -25,7 +25,7 @@ def test_stream_agent_unauthorized():
     assert response.status_code == 401
 
 @pytest.mark.asyncio
-async def test_stream_agent_success(auth_header):
+async def test_stream_agent_success(auth_header, mock_user):
     """Test successful agent streaming with SSE."""
     # We need to mock the LangGraph app.astream_events
     mock_events = [
@@ -39,10 +39,18 @@ async def test_stream_agent_success(auth_header):
             yield event
 
     with patch("ea_chatbot.api.routers.agent.app.astream_events", side_effect=mock_astream_events), \
-         patch("ea_chatbot.api.routers.agent.get_checkpointer") as mock_cp:
+         patch("ea_chatbot.api.routers.agent.get_checkpointer") as mock_cp, \
+         patch("ea_chatbot.api.routers.agent.history_manager") as mock_hm:
         
         mock_cp.return_value.__aenter__.return_value = AsyncMock()
         
+        # Mock session and DB objects
+        mock_session = MagicMock()
+        mock_hm.get_session.return_value.__enter__.return_value = mock_session
+        from ea_chatbot.history.models import Conversation
+        mock_conv = Conversation(id="t1", user_id=mock_user.id)
+        mock_session.get.return_value = mock_conv
+        
         # Using TestClient with a stream context
         with client.stream("POST", "/chat/stream", 
                           json={"message": "hello", "thread_id": "t1"},