Minor session messages optimization (#935)

Cache Session.messages as an UnmodifiableListView and refresh it only
when the underlying map changes, instead of recreating a new
unmodifiable list on every getter call. This keeps the data immutable
but gives widgets a stable reference, reducing unnecessary rebuilds.

Author: hiroshihorie

lib/src/agent/session.dart

@@ -156,8 +156,9 @@ class Session extends DisposableChangeNotifier {
       };
 
   final LinkedHashMap<String, ReceivedMessage> _messages = LinkedHashMap();
+  UnmodifiableListView<ReceivedMessage> _messagesView = UnmodifiableListView<ReceivedMessage>(const []);
 
-  List<ReceivedMessage> get messages => List.unmodifiable(_messages.values);
+  List<ReceivedMessage> get messages => _messagesView;
 
   final List<MessageSender> _senders = [];
   final List<MessageReceiver> _receivers = [];
@@ -269,6 +270,7 @@ class Session extends DisposableChangeNotifier {
               (message) => MapEntry(message.id, message),
             ),
       );
+    _updateMessagesView();
     notifyListeners();
   }
 
@@ -288,6 +290,7 @@ class Session extends DisposableChangeNotifier {
           final shouldNotify = existing != message;
           _messages[message.id] = message;
           if (shouldNotify) {
+            _updateMessagesView();
             notifyListeners();
           }
         },
@@ -300,6 +303,10 @@ class Session extends DisposableChangeNotifier {
     }
   }
 
+  void _updateMessagesView() {
+    _messagesView = UnmodifiableListView(_messages.values.toList(growable: false));
+  }
+
   Future<void> _cancelReceiverSubscriptions() async {
     for (final subscription in _receiverSubscriptions) {
       await subscription.cancel();