Refactor: nearly done phase 5

core/communication/messages.py

@@ -3,7 +3,7 @@ Message types, constants, and validation functions for WebSocket communication.
 """
 import json
 import logging
-from typing import Dict, Any, Optional
+from typing import Dict, Any, Optional, Union
 from .models import (
     IncomingMessage, OutgoingMessage,
     SetSubscriptionListMessage, SetSessionIdMessage, SetProgressionStageMessage,
@@ -161,14 +161,14 @@ def create_state_report(cpu_usage: float, memory_usage: float,
     )
 
 
-def create_image_detection(subscription_identifier: str, detection_data: Dict[str, Any],
+def create_image_detection(subscription_identifier: str, detection_data: Union[Dict[str, Any], None],
                           model_id: int, model_name: str) -> ImageDetectionMessage:
     """
     Create an image detection message.
 
     Args:
         subscription_identifier: Camera subscription identifier
-        detection_data: Flat dictionary of detection results
+        detection_data: Detection results - Dict for data, {} for empty, None for abandonment
         model_id: Model identifier
         model_name: Model name
 
@@ -176,6 +176,12 @@ def create_image_detection(subscription_identifier: str, detection_data: Dict[st
         ImageDetectionMessage object
     """
     from .models import DetectionData
+    from typing import Union
+
+    # Handle three cases:
+    # 1. None = car abandonment (detection: null)
+    # 2. {} = empty detection (triggers session creation)
+    # 3. {...} = full detection data (updates session)
 
     data = DetectionData(
         detection=detection_data,