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.lock

@@ -790,7 +790,7 @@ packages:
     source: hosted
     version: "1.3.0"
   markdown:
-    dependency: transitive
+    dependency: "direct main"
     description:
       name: markdown
       sha256: "935e23e1ff3bc02d390bad4d4be001208ee92cc217cb5b5a6c19bc14aaa318c1"