refactor: migrate to flutter_markdown_plus for enhanced markdown support

- Replaced the gpt_markdown package with flutter_markdown_plus to improve markdown rendering capabilities.
- Updated the markdown configuration to utilize the new package, enhancing the overall maintainability and adaptability of markdown features.
- Adjusted the streaming markdown widget to integrate with the new markdown body implementation, ensuring a cohesive user experience.
- Removed deprecated builders and streamlined the code for better clarity and performance.

lib/features/chat/widgets/assistant_message_widget.dart

@@ -768,6 +768,7 @@ class _AssistantMessageWidgetState extends ConsumerState<AssistantMessageWidget>
     Widget buildDefault(BuildContext context) => StreamingMarkdownWidget(
       content: processedContent,
       isStreaming: widget.isStreaming,
+      onTapLink: (url, _) => _launchUri(url),
     );
 
     final responseBuilder = ref.watch(assistantResponseBuilderProvider);