fix: match user_order (singular) message type from API

The Kalshi WS API sends type="user_order" (singular) for order updates,
not "user_orders". Add the singular form to the routing match arm.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: jaredmcqueen

src/ws/message.rs

@@ -732,7 +732,7 @@ impl StreamMessage {
             "order_group_updates" => serde_json::from_value::<OrderGroupUpdateData>(value)
                 .map(StreamMessage::OrderGroupUpdate),
             // User order update notifications
-            "user_orders" => serde_json::from_value::<UserOrderData>(value)
+            "user_order" | "user_orders" => serde_json::from_value::<UserOrderData>(value)
                 .map(|d| StreamMessage::UserOrder(Box::new(d))),
             // Multivariate lookup notifications
             "multivariate_lookup" => serde_json::from_value::<MultivariateLookupData>(value)