feat(spawners): add typed JSON binding for ProximitySpawner

- Add ProximitySpawner.Json.cs with shadow properties for triggerRange,
  spawnMessage, and instant; OnAfterJsonDeserialize calls base then applies them
- Mark ProximitySpawner [JsonDiscoverableType] and its parameterless ctor
  [JsonConstructor] for STJ polymorphism discovery
- Add ProximitySpawnerRoundTripTests with test-hygiene finally cleanup

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

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

@@ -0,0 +1,41 @@
+using System.Collections.Generic;
+using System.Text.Json;
+using Server;
+using Server.Engines.Spawners;
+using Xunit;
+
+namespace UOContent.Tests.Engines.Spawners.Json;
+
+[Collection("Sequential UOContent Tests")]
+public class ProximitySpawnerRoundTripTests
+{
+    [Fact]
+    public void ProximitySpawner_RoundTrips_ProximityFields()
+    {
+        ProximitySpawner spawner = null;
+        ProximitySpawner s = null;
+        try
+        {
+            spawner = new ProximitySpawner("Fisherman") { TriggerRange = 4, InstantFlag = true };
+            spawner.SpawnMessage = 500000;
+            spawner.MoveToWorld(new Point3D(120, 120, 0), Map.Felucca);
+
+            var json = JsonSerializer.Serialize<List<BaseSpawner>>(
+                new List<BaseSpawner> { spawner }, SpawnerJsonSerializer.Options);
+            Assert.Contains("\"$type\": \"ProximitySpawner\"", json);
+            Assert.Contains("\"triggerRange\": 4", json);
+            Assert.Contains("\"instant\": true", json);
+
+            var rt = JsonSerializer.Deserialize<List<BaseSpawner>>(json, SpawnerJsonSerializer.Options);
+            s = Assert.IsType<ProximitySpawner>(Assert.Single(rt));
+            Assert.Equal(4, s.TriggerRange);
+            Assert.True(s.InstantFlag);
+            Assert.Equal(500000, s.SpawnMessage.Number);
+        }
+        finally
+        {
+            s?.Delete();
+            spawner?.Delete();
+        }
+    }
+}

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

@@ -0,0 +1,58 @@
+/*************************************************************************
+ * ModernUO                                                              *
+ * Copyright 2019-2026 - ModernUO Development Team                       *
+ * Email: hi@modernuo.com                                                *
+ * File: ProximitySpawner.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;
+
+namespace Server.Engines.Spawners;
+
+public partial class ProximitySpawner
+{
+    private int _jsonTriggerRange;
+    private TextDefinition _jsonSpawnMessage;
+    private bool _jsonInstant;
+
+    [JsonInclude]
+    [JsonPropertyName("triggerRange")]
+    public int JsonTriggerRange
+    {
+        get => TriggerRange;
+        set => _jsonTriggerRange = value;
+    }
+
+    [JsonInclude]
+    [JsonPropertyName("spawnMessage")]
+    public TextDefinition JsonSpawnMessage
+    {
+        get => SpawnMessage;
+        set => _jsonSpawnMessage = value;
+    }
+
+    [JsonInclude]
+    [JsonPropertyName("instant")]
+    public bool JsonInstant
+    {
+        get => InstantFlag;
+        set => _jsonInstant = value;
+    }
+
+    protected internal override void OnAfterJsonDeserialize()
+    {
+        base.OnAfterJsonDeserialize();
+
+        TriggerRange = _jsonTriggerRange;
+        SpawnMessage = _jsonSpawnMessage;
+        InstantFlag = _jsonInstant;
+    }
+}

Projects/UOContent/Engines/Spawners/ProximitySpawner.cs

@@ -22,6 +22,7 @@ using Server.Mobiles;
 namespace Server.Engines.Spawners;
 
 [SerializationGenerator(0)]
+[JsonDiscoverableType]
 public partial class ProximitySpawner : Spawner
 {
     [SerializableField(0)]
@@ -37,6 +38,7 @@ public partial class ProximitySpawner : Spawner
     private bool _instantFlag;
 
     [Constructible(AccessLevel.Developer)]
+    [System.Text.Json.Serialization.JsonConstructor]
     public ProximitySpawner()
     {
     }