fix(UnionJsonConverter): Fix fallback logic when deserialization fails

When deserializing a union type, if parsing of the preferred type fails, it should fall back to the alternative type. Added the TryDeserialize method for safe parsing attempts, and adjusted the priority judgment between enum and string types in ShouldChooseFirst to ensure correct execution of the fallback logic.

Author: Yushu2606

src/Utils/Union.cs

@@ -148,16 +148,29 @@ public override Union<T1, T2> Read(ref Utf8JsonReader reader, Type typeToConvert
                 return new(null);
             }
 
+            Utf8JsonReader originalReader = reader;
             bool preferFirst = TryChooseFirstAdvanced(typeof(T1), typeof(T2), ref reader, options)
                                ?? ShouldChooseFirst(typeof(T1), typeof(T2), reader.TokenType);
             if (preferFirst)
             {
-                T1? first = JsonSerializer.Deserialize<T1>(ref reader, options);
-                return new(first);
+                if (TryDeserialize(ref reader, options, out T1? first))
+                {
+                    return new(first);
+                }
+
+                reader = originalReader;
+                T2? secondFallback = JsonSerializer.Deserialize<T2>(ref reader, options);
+                return new(secondFallback);
+            }
+
+            if (TryDeserialize(ref reader, options, out T2? second))
+            {
+                return new(second);
             }
 
-            T2? second = JsonSerializer.Deserialize<T2>(ref reader, options);
-            return new(second);
+            reader = originalReader;
+            T1? firstFallback = JsonSerializer.Deserialize<T1>(ref reader, options);
+            return new(firstFallback);
         }
 
         public override void Write(Utf8JsonWriter writer, Union<T1, T2> value, JsonSerializerOptions options)
@@ -322,8 +335,18 @@ private static bool ShouldChooseFirst(Type t1, Type t2, JsonTokenType kind)
             {
                 case JsonTokenType.String:
                     {
-                        bool t1Match = IsStringLike(t1);
                         bool t2Match = IsStringLike(t2);
+                        if (t1.IsEnum && t2Match)
+                        {
+                            return true;
+                        }
+
+                        bool t1Match = IsStringLike(t1);
+                        if (t2.IsEnum && t1Match)
+                        {
+                            return false;
+                        }
+
                         return t1Match switch
                         {
                             true when !t2Match => true,
@@ -387,6 +410,27 @@ private static bool ShouldChooseFirst(Type t1, Type t2, JsonTokenType kind)
             }
         }
 
+        private static bool TryDeserialize<T>(ref Utf8JsonReader reader, JsonSerializerOptions options, out T? value)
+        {
+            Utf8JsonReader copy = reader;
+            try
+            {
+                value = JsonSerializer.Deserialize<T>(ref copy, options);
+                reader = copy;
+                return true;
+            }
+            catch (JsonException)
+            {
+                value = default;
+                return false;
+            }
+            catch (NotSupportedException)
+            {
+                value = default;
+                return false;
+            }
+        }
+
         private static Type UnwrapNullable(Type t) => Nullable.GetUnderlyingType(t) ?? t;
 
         private static bool IsDictionaryLike(Type t)