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.

lib/features/chat/providers/assistant_response_builder_provider.dart

@@ -0,0 +1,25 @@
+import 'package:flutter/widgets.dart';
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+
+import '../../../core/models/chat_message.dart';
+
+typedef AssistantResponseBuilder =
+    Widget Function(BuildContext context, AssistantResponseContext response);
+
+class AssistantResponseContext {
+  const AssistantResponseContext({
+    required this.message,
+    required this.markdown,
+    required this.isStreaming,
+    required this.buildDefault,
+  });
+
+  final ChatMessage message;
+  final String markdown;
+  final bool isStreaming;
+  final WidgetBuilder buildDefault;
+}
+
+final assistantResponseBuilderProvider = Provider<AssistantResponseBuilder?>(
+  (_) => null,
+);