Added Artifact Handling for Chat Messages

- Enhanced the `Chat` model to include an `artifacts` field to hold associated artifacts.
- Updated the `AgentMessageTile` widget to display the number of artifacts and added functionality to trigger artifact downloads upon button press.
- Introduced a method in `ChatViewModel` to leverage the `ChatService` for artifact downloads.

frontend/lib/models/step.dart

@@ -8,6 +8,7 @@ class Step {
   final String status;
   final String output;
   final Map<String, dynamic> additionalOutput;
+  // TODO: Create an actual artifact object
   final List<dynamic> artifacts;
   final bool isLast;