feat: Add support for temporary chat message

- Added logic to display a temporary chat message while waiting for the response from the agent.
- When a user sends a message, a temporary chat message is created and displayed.
- Implemented a method to remove the temporary chat message when the response is received or in case of an error.
- Updated the UI to invoke the method for adding a temporary chat message when the send button is pressed.

Feedback:
- This change improves the user experience by allowing them to see their own message immediately while waiting for the response from the agent.

frontend/lib/viewmodels/chat_viewmodel.dart

@@ -95,7 +95,9 @@ class ChatViewModel with ChangeNotifier {
       }
 
       // Assign the chats list
-      _chats = chats;
+      if (chats.length > 0) {
+        _chats = chats;
+      }
 
       // Notify listeners to rebuild UI
       notifyListeners();
@@ -154,6 +156,9 @@ class ChatViewModel with ChangeNotifier {
 
       _chats.add(agentChat);
 
+      // Remove the temporary message
+      removeTemporaryMessage();
+
       // Notify UI of the new chats
       notifyListeners();
 
@@ -170,6 +175,8 @@ class ChatViewModel with ChangeNotifier {
 
       print("Chats added for task ID: $_currentTaskId");
     } catch (e) {
+      // Remove the temporary message in case of an error
+      removeTemporaryMessage();
       // TODO: We are bubbling up the full response. Revisit this.
       rethrow;
       // TODO: Handle additional error scenarios or log them as required
@@ -179,6 +186,25 @@ class ChatViewModel with ChangeNotifier {
     }
   }
 
+  void addTemporaryMessage(String message) {
+    Chat tempMessage = Chat(
+        // You can generate a unique ID or use a placeholder
+        id: "TEMP_ID",
+        taskId: "TEMP_ID",
+        message: message,
+        timestamp: DateTime.now(),
+        messageType: MessageType.user,
+        artifacts: []);
+
+    _chats.add(tempMessage);
+    notifyListeners();
+  }
+
+  void removeTemporaryMessage() {
+    _chats.removeWhere((chat) => chat.id == "TEMP_ID");
+    notifyListeners();
+  }
+
   /// Downloads an artifact associated with a specific chat.
   ///
   /// [taskId] is the ID of the task.

frontend/lib/views/chat/chat_view.dart

@@ -113,6 +113,7 @@ class _ChatViewState extends State<ChatView> {
             padding: const EdgeInsets.all(8.0),
             child: ChatInputField(
               onSendPressed: (message) async {
+                widget.viewModel.addTemporaryMessage(message);
                 try {
                   if (widget.viewModel.currentTaskId != null) {
                     widget.viewModel.sendChatMessage(