feat(spawners): add typed JSON binding for RegionSpawner

Add RegionSpawner.Json.cs with a `region` shadow property (serializes
SpawnRegion.Name) and OnAfterJsonDeserialize override that resolves the
region by name using _jsonMap from BaseSpawner. Mark RegionSpawner with
[JsonDiscoverableType] and [JsonConstructor] on its parameterless ctor
so SpawnerJsonSerializer discovers it for STJ polymorphism.

Add RegionSpawnerRoundTripTests: create+register a test BaseRegion on
Felucca (game regions are not loaded in the test environment), serialize
a RegionSpawner, assert $type and region name in JSON, deserialize, and
assert SpawnRegion.Name round-trips correctly. RED → GREEN confirmed.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Projects/UOContent.Tests/Tests/Engines/Spawners/Json/RegionSpawnerRoundTripTests.cs

@@ -0,0 +1,48 @@
+using System.Collections.Generic;
+using System.Text.Json;
+using Server;
+using Server.Engines.Spawners;
+using Server.Regions;
+using Xunit;
+
+namespace UOContent.Tests.Engines.Spawners.Json;
+
+[Collection("Sequential UOContent Tests")]
+public class RegionSpawnerRoundTripTests
+{
+    [Fact]
+    public void RegionSpawner_RoundTrips_RegionByName()
+    {
+        // The test environment does not load game regions (no AssemblyHandler.Invoke("Initialize")),
+        // so we create and register a test BaseRegion on Felucca directly.
+        var region = new BaseRegion(
+            "TestSpawnRegion",
+            Map.Felucca,
+            50,
+            new Rectangle3D(1400, 1670, -128, 40, 40, 256)
+        );
+        region.Register();
+
+        try
+        {
+            var spawner = new RegionSpawner("Fisherman") { SpawnRegion = region };
+            spawner.MoveToWorld(new Point3D(1416, 1683, 0), Map.Felucca);
+
+            var json = JsonSerializer.Serialize<List<BaseSpawner>>(
+                new List<BaseSpawner> { spawner }, SpawnerJsonSerializer.Options);
+            Assert.Contains("\"$type\": \"RegionSpawner\"", json);
+            Assert.Contains(region.Name, json);
+
+            var rt = JsonSerializer.Deserialize<List<BaseSpawner>>(json, SpawnerJsonSerializer.Options);
+            var s = Assert.IsType<RegionSpawner>(Assert.Single(rt));
+            Assert.Equal(region.Name, s.SpawnRegion?.Name);
+
+            s.Delete();
+            spawner.Delete();
+        }
+        finally
+        {
+            region.Unregister();
+        }
+    }
+}

Projects/UOContent/Engines/Spawners/RegionSpawner.Json.cs

@@ -0,0 +1,41 @@
+/*************************************************************************
+ * ModernUO                                                              *
+ * Copyright 2019-2026 - ModernUO Development Team                       *
+ * Email: hi@modernuo.com                                                *
+ * File: RegionSpawner.Json.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.Text.Json.Serialization;
+using Server.Regions;
+
+namespace Server.Engines.Spawners;
+
+public partial class RegionSpawner
+{
+    private string _jsonRegion;
+
+    [JsonInclude]
+    [JsonPropertyName("region")]
+    public string JsonRegion
+    {
+        get => SpawnRegion?.Name;
+        set => _jsonRegion = value;
+    }
+
+    protected internal override void OnAfterJsonDeserialize()
+    {
+        base.OnAfterJsonDeserialize();
+
+        _spawnRegion = Region.Find(_jsonRegion, _jsonMap) as BaseRegion;
+        _spawnRegion?.InitRectangles();
+        SpawnRegionName = _spawnRegion?.Name;
+    }
+}

Projects/UOContent/Engines/Spawners/RegionSpawner.cs

@@ -22,6 +22,7 @@ using Server.Regions;
 namespace Server.Engines.Spawners;
 
 [SerializationGenerator(0)]
+[JsonDiscoverableType]
 public partial class RegionSpawner : Spawner
 {
     [SerializableField(0, getter: "private", setter: "private")]
@@ -32,6 +33,7 @@ public partial class RegionSpawner : Spawner
     public override Region Region => _spawnRegion;
 
     [Constructible(AccessLevel.Developer)]
+    [System.Text.Json.Serialization.JsonConstructor]
     public RegionSpawner()
     {
     }