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

src/Tests/HtmlAgilityPack.Tests.NetStandard2_0/HtmlEntityDeEntitizeTests.cs

@@ -0,0 +1,25 @@
+using Xunit;
+
+namespace HtmlAgilityPack.Tests.NetStandard2_0
+{
+    public class HtmlEntityDeEntitizeTests
+    {
+        [Theory]
+        // Invalid numeric entities must be left untouched; "&#foo;" used to become
+        // "&##foo;".
+        [InlineData("&#foo;", "&#foo;")]
+        [InlineData("&#xGG;", "&#xGG;")]
+        [InlineData("&#;", "&#;")]
+        [InlineData("prefix &#foo; suffix", "prefix &#foo; suffix")]
+        // Valid numeric entities keep decoding correctly.
+        [InlineData("A", "A")]
+        [InlineData("A", "A")]
+        // Named entities are unaffected.
+        [InlineData("&", "&")]
+        [InlineData("&notreal;", "&notreal;")]
+        public void DeEntitize_HandlesInvalidNumericEntitiesWithoutDuplicatingHash(string input, string expected)
+        {
+            Assert.Equal(expected, HtmlEntity.DeEntitize(input));
+        }
+    }
+}