update session ID handling in worker communication protocol; allow null session ID to indicate no active session

worker.md

@@ -257,7 +257,7 @@ Backend's response to a `patchSession` message.
 
 ### 5.5. Set Session ID
 
-Allows the backend to instruct the worker to associate a session ID with a subscription. This is useful for linking detection events to a specific session.
+Allows the backend to instruct the worker to associate a session ID with a subscription. This is useful for linking detection events to a specific session. The session ID can be `null` to indicate no active session.
 
 - **Type:** `setSessionId`
 
@@ -273,8 +273,20 @@ Allows the backend to instruct the worker to associate a session ID with a subsc
 }
 ```
 
+Or to clear the session:
+
+```json
+{
+  "type": "setSessionId",
+  "payload": {
+    "subscriptionIdentifier": "display-001;cam-002",
+    "sessionId": null
+  }
+}
+```
+
 > **Note:**
-> - The worker should store the session ID for the given subscription and use it in subsequent detection or patch messages as appropriate.
+> - The worker should store the session ID for the given subscription and use it in subsequent detection or patch messages as appropriate. If `sessionId` is `null`, the worker should treat the subscription as having no active session.
 
 ## 6. Example Communication Log