test(spawners): assert all migrated spawn files build via DTO

Regression test that deserializes every spawned JSON file under
Data/Spawns/ via SpawnerDto, calls ToSpawner() to build the live
spawner, and deletes it to prevent world leaks. Catches both JSON
deserialization failures and exceptions during spawner construction,
collecting failures per file. Any failure names the offending file
and is a real migration defect, not a test weakness.

All 484 tests pass including the new AllSpawnFilesLoadTests.

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

@@ -0,0 +1,72 @@
+// Regression test: verifies that EVERY migrated spawn file deserializes and builds spawners.
+// This catches any migration defects where a spawn file is syntactically correct JSON
+// but fails to build spawners via DTO.ToSpawner(). Any failure names the offending file
+// and is a real migration defect, not a test weakness.
+
+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Text.Json;
+using Server;
+using Server.Engines.Spawners;
+using Xunit;
+
+namespace UOContent.Tests.Engines.Spawners.Json;
+
+[Collection("Sequential UOContent Tests")]
+public class AllSpawnFilesLoadTests
+{
+    [Fact]
+    public void EverySpawnFile_DeserializesAndBuildsSpawners()
+    {
+        var root = Path.Combine(Core.BaseDirectory, "Data", "Spawns");
+        if (!Directory.Exists(root))
+        {
+            return; // distribution data not present in this checkout
+        }
+
+        var failures = new List<string>();
+        var fileCount = 0;
+        var spawnerCount = 0;
+
+        foreach (var file in Directory.EnumerateFiles(root, "*.json", SearchOption.AllDirectories))
+        {
+            fileCount++;
+            try
+            {
+                var dtos = JsonSerializer.Deserialize<List<SpawnerDto>>(
+                    File.ReadAllText(file), SpawnerJsonSerializer.Options);
+
+                if (dtos != null)
+                {
+                    foreach (var dto in dtos)
+                    {
+                        try
+                        {
+                            var spawner = dto.ToSpawner();
+                            spawnerCount++;
+                            spawner?.Delete();
+                        }
+                        catch (Exception ex)
+                        {
+                            failures.Add($"{file}: ToSpawner failed: {ex.Message}");
+                        }
+                    }
+                }
+            }
+            catch (JsonException ex)
+            {
+                failures.Add($"{file}: JSON deserialization failed: {ex.Message}");
+            }
+            catch (Exception ex)
+            {
+                failures.Add($"{file}: {ex.GetType().Name}: {ex.Message}");
+            }
+        }
+
+        Assert.True(
+            failures.Count == 0,
+            $"Loaded {fileCount} files, built {spawnerCount} spawners. Failures:\n{string.Join("\n", failures)}"
+        );
+    }
+}