feat(json): add reusable JsonDiscoverableType opt-in marker attribute

Projects/Server.Tests/Tests/Json/JsonDiscoverableTypeAttributeTests.cs

@@ -0,0 +1,30 @@
+using Server.Json;
+using Xunit;
+
+namespace Server.Tests.Json;
+
+public class JsonDiscoverableTypeAttributeTests
+{
+    [JsonDiscoverableType]
+    private sealed class DefaultName { }
+
+    [JsonDiscoverableType("custom")]
+    private sealed class Overridden { }
+
+    [Fact]
+    public void DefaultDiscriminator_IsNull()
+    {
+        var attr = (JsonDiscoverableTypeAttribute)System.Attribute.GetCustomAttribute(
+            typeof(DefaultName), typeof(JsonDiscoverableTypeAttribute), false);
+        Assert.NotNull(attr);
+        Assert.Null(attr.Discriminator);
+    }
+
+    [Fact]
+    public void OverrideDiscriminator_IsReturned()
+    {
+        var attr = (JsonDiscoverableTypeAttribute)System.Attribute.GetCustomAttribute(
+            typeof(Overridden), typeof(JsonDiscoverableTypeAttribute), false);
+        Assert.Equal("custom", attr.Discriminator);
+    }
+}

Projects/Server/Json/JsonDiscoverableTypeAttribute.cs

@@ -0,0 +1,32 @@
+/*************************************************************************
+ * ModernUO                                                              *
+ * Copyright 2019-2026 - ModernUO Development Team                       *
+ * Email: hi@modernuo.com                                                *
+ * File: JsonDiscoverableTypeAttribute.cs                                *
+ *                                                                       *
+ * This program is free software: you can redistribute it and/or modify  *
+ * it under the terms of the GNU General Public License as published by  *
+ * the Free Software Foundation, either version 3 of the License, or     *
+ * (at your option) any later version.                                   *
+ *                                                                       *
+ * You should have received a copy of the GNU General Public License     *
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>. *
+ *************************************************************************/
+
+using System;
+
+namespace Server.Json;
+
+/// <summary>
+/// Marks a concrete class as a discoverable polymorphic JSON derived type. A consumer
+/// (e.g. <c>SpawnerJsonSerializer</c>) scans assemblies for marked subclasses of a chosen
+/// base and registers them for System.Text.Json polymorphism. Optionally overrides the
+/// <c>$type</c> discriminator value (defaults to the type's <see cref="System.Type.Name"/>).
+/// </summary>
+[AttributeUsage(AttributeTargets.Class, Inherited = false)]
+public sealed class JsonDiscoverableTypeAttribute : Attribute
+{
+    public JsonDiscoverableTypeAttribute(string discriminator = null) => Discriminator = discriminator;
+
+    public string Discriminator { get; }
+}