feat(spawners): virtual OnTick; SpawnerDto carries the group flag (#2640)

## Summary

Two small additive changes a derived spawner needs.

- **`BaseSpawner.OnTick()` is now `virtual`.** A subclass that gates spawning on external state (time windows, event triggers) must gate *timer* spawns without gating the manual `Spawn()` API, and `OnTick` is the only place the two paths differ: it is the timer callback and `Spawn()` is both what it calls and what commands and scripts call. Cost: one virtual dispatch on the existing timer callback; no change to stock behaviour.
- **`group` in the JSON DTO.** `BaseSpawner.Group` (all dead, then respawn) is binary-persisted but was missing from `SpawnerDto`, so it did not survive export/import. Added to the abstract record after `spawnLocationIsHome`, assigned in `ApplyDto` after `InitSpawn` (which resets it), and exported by the three stock `ToDto` implementations.

## Test plan

- [x] A derived spawner overriding `OnTick` with a closed gate: `OnTick()` spawns nothing; manual `Spawn()` still spawns and does not pass through `OnTick`.
- [x] DTO round trip with `Group = true` carries `group` and restores it on `ToSpawner()`.
- [x] `UOContent.Tests` full suite green.
- [ ] CI

Projects/UOContent.Tests/Tests/Engines/Spawners/SpawnerTickAndGroupDtoTests.cs

@@ -0,0 +1,78 @@
+using System;
+using System.Collections.Generic;
+using System.Text.Json;
+using Server.Engines.Spawners;
+using Xunit;
+
+namespace Server.Tests.Engines.Spawners;
+
+[Collection("Sequential UOContent Tests")]
+public class SpawnerTickAndGroupDtoTests
+{
+    private sealed class GatedSpawner : Spawner
+    {
+        public int Ticks;
+        public bool Gate = false;
+
+        public GatedSpawner() : base(1, TimeSpan.FromMinutes(5), TimeSpan.FromMinutes(10), 0, default, "Rabbit")
+        {
+        }
+
+        public GatedSpawner(Serial serial) : base(serial)
+        {
+        }
+
+        public override void OnTick()
+        {
+            Ticks++;
+            if (Gate)
+            {
+                base.OnTick();
+            }
+        }
+    }
+
+    [Fact]
+    public void OnTick_IsVirtual_AndManualSpawnBypassesIt()
+    {
+        var spawner = new GatedSpawner();
+        spawner.MoveToWorld(new Point3D(1500, 1500, 0), Map.Felucca);
+        try
+        {
+            spawner.OnTick();
+            Assert.Equal(1, spawner.Ticks);
+            Assert.Empty(spawner.Spawned);      // gate closed: base.OnTick not reached
+
+            spawner.Spawn();                    // manual API does not go through OnTick
+            Assert.Equal(1, spawner.Ticks);
+            Assert.Single(spawner.Spawned);
+        }
+        finally
+        {
+            spawner.Delete();
+        }
+    }
+
+    [Fact]
+    public void Dto_RoundTrip_CarriesGroup()
+    {
+        var spawner = new Spawner(1, TimeSpan.FromMinutes(5), TimeSpan.FromMinutes(10), 0, default, "Rabbit");
+        spawner.MoveToWorld(new Point3D(1500, 1500, 0), Map.Felucca);
+        Spawner loaded = null;
+        try
+        {
+            spawner.Group = true;
+            var json = SpawnerJsonSerializer.SerializeCompact(new List<SpawnerDto> { spawner.ToDto() });
+            Assert.Contains("\"group\"", json);
+
+            var dtos = JsonSerializer.Deserialize<List<SpawnerDto>>(json, SpawnerJsonSerializer.Options);
+            loaded = (Spawner)dtos[0].ToSpawner();
+            Assert.True(loaded.Group);
+        }
+        finally
+        {
+            loaded?.Delete();
+            spawner.Delete();
+        }
+    }
+}