feat(reasoning): Add html_unescape and enhance reasoning parser

lib/core/utils/html_utils.dart

@@ -0,0 +1,18 @@
+/// HTML entity utilities for parsing content.
+///
+/// Reference: openwebui-src/src/lib/utils/index.ts (unescapeHtml)
+library;
+
+import 'package:html_unescape/html_unescape.dart';
+
+/// Utility class for HTML entity handling.
+class HtmlUtils {
+  /// HTML entity unescaper instance.
+  static final _unescape = HtmlUnescape();
+
+  /// Unescape HTML entities in a string.
+  ///
+  /// Handles all Named, Decimal, and Hexadecimal Character References.
+  static String unescapeHtml(String s) => _unescape.convert(s);
+}
+