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/core/utils/markdown_stream_formatter.dart

@@ -0,0 +1,67 @@
+/// Maintains a raw markdown buffer for streaming content and generates
+/// preview-safe output by appending synthetic closing tokens when necessary.
+class MarkdownStreamFormatter {
+  StringBuffer _raw = StringBuffer();
+
+  /// Seeds the formatter with existing markdown content.
+  void seed(String content) {
+    _raw = StringBuffer(content);
+  }
+
+  /// Adds a streaming chunk to the internal buffer and returns a preview-ready
+  /// string with any required synthetic closing markers.
+  String ingest(String chunk) {
+    if (chunk.isNotEmpty) {
+      _raw.write(chunk);
+    }
+    return preview();
+  }
+
+  /// Replaces the current buffer with the provided [content].
+  String replace(String content) {
+    seed(content);
+    return preview();
+  }
+
+  /// Returns the preview-safe markdown string.
+  String preview() {
+    final raw = _raw.toString();
+    return raw + _syntheticClosures(raw);
+  }
+
+  /// Returns the raw markdown accumulated so far.
+  String finalize() => _raw.toString();
+
+  String _syntheticClosures(String content) {
+    final buffer = StringBuffer();
+
+    final fenceCount = '```'.allMatches(content).length;
+    if (fenceCount.isOdd) {
+      buffer.writeln('```');
+    }
+
+    final boldCount = RegExp(r'\*\*').allMatches(content).length;
+    if (boldCount.isOdd) {
+      buffer.write('**');
+    }
+
+    final italicCount = RegExp(r'(?<!\*)\*(?!\*)').allMatches(content).length;
+    if (italicCount.isOdd) {
+      buffer.write('*');
+    }
+
+    final openBrackets = '['.allMatches(content).length;
+    final closeBrackets = ']'.allMatches(content).length;
+    if (openBrackets > closeBrackets) {
+      buffer.write(List.filled(openBrackets - closeBrackets, ']').join());
+    }
+
+    final openParens = '('.allMatches(content).length;
+    final closeParens = ')'.allMatches(content).length;
+    if (openParens > closeParens) {
+      buffer.write(List.filled(openParens - closeParens, ')').join());
+    }
+
+    return buffer.toString();
+  }
+}