change session_id to String in WebSocketInitMessage to match frontend type

Author: itang06

backend/websocket-server/src/main.rs

@@ -19,7 +19,7 @@ use serde_json;
 
 #[derive(Deserialize)]
 struct WebSocketInitMessage {
-    session_id: i32,
+    session_id: String,
     nodes: Vec<Node>,
 }