Fix HtmlEntity.DeEntitize duplicating '#' on invalid numeric entities

When a numeric character reference (&#...;) fails to parse, the catch block
restored the text as "&#" + e where e already begins with '#', producing a
doubled '#' (for example "&#foo;" became "&##foo;"). Restore the original
text as "&" + e + ";" to match the named-entity fallback and keep DeEntitize
idempotent for unparseable numeric entities.

src/HtmlAgilityPack.Shared/HtmlEntity.cs

@@ -663,7 +663,8 @@ namespace HtmlAgilityPack
                                         }
                                         catch
                                         {
-                                            sb.Append("&#" + e + ";");
+                                            // 'e' already starts with '#', so prepending "&#" would duplicate it.
+                                            sb.Append("&" + e + ";");
                                         }
                                     }
                                     else