fix(mcp): prevent internal handler serialization errors from being silently discarded or defaulting to empty strings

server/src/api/ws.rs

@@ -86,7 +86,7 @@ pub async fn handle_socket(socket: WebSocket, state: Arc<AppState>, _client_type
                     if let Ok(payload) = serde_json::from_str::<serde_json::Value>(&text) {
                         // Process MCP request
                         if let Some(response) = handler.handle_request(payload).await {
-                            let res_str = serde_json::to_string(&response).unwrap_or_default();
+                            let res_str = serde_json::to_string(&response).unwrap_or_else(|e| format!(r#"{{\"jsonrpc\":\"2.0\",\"id\":null,\"error\":{{\"code\":-32603,\"message\":\"{}\"}}}}"#, e));
                             let tx_opt = state_clone
                                 .clients
                                 .read()