chore: update markdown dependency and refactor streaming handling

- Added `markdown` dependency version `^7.2.1` in `pubspec.yaml`.
- Updated `pubspec.lock` to reflect the direct dependency change.
- Refactored `streaming_helper.dart` to utilize `StreamingResponseController` for better stream management.
- Enhanced `ChatMessagesNotifier` to handle message streams with improved formatting and error handling.
- Updated `StreamingMarkdownWidget` to streamline markdown rendering and support new configurations.

pubspec.yaml

@@ -29,6 +29,7 @@ dependencies:
   
   # UI Components - Enhanced Markdown  
   flutter_markdown_plus: ^1.0.5
+  markdown: ^7.2.1
   cached_network_image: ^3.3.1
   socket_io_client: ^3.1.2
   yaml: ^3.1.2