fix(tests): single shared bootstrap; idempotent SerializationThreadWorker.Exit (#2473)

## Problem

Running the full `UOContent.Tests` suite, the test host **hangs ~2.5 minutes at shutdown and then crashes** (`Test host process crashed` / run aborted). The tests themselves are fine — they complete in ~1s — but the process can't exit.

Captured via `--blame-hang` dump. The blocking thread:

```
System.Threading.WaitHandle.WaitOne()
Server.SerializationThreadWorker.Sleep()        SerializationThreadWorker.cs:54  (_stopEvent.WaitOne())
Server.SerializationThreadWorker.Exit()         SerializationThreadWorker.cs:61
Server.World.ExitSerializationThreads()         World.cs:429
Server.Tests.UOContentFixture..ctor()
```

### Root cause

Both collection fixtures (`UOContentFixture` and `PathfindingTestFixture`) each run the **full process-global ModernUO bootstrap**. `World.Load()` is guarded to run once per process, so the **second** fixture's `World.Load()` is a no-op and does **not** respawn the serialization workers — but `World.ExitSerializationThreads()` is **not** guarded, so the second fixture calls `Exit()` on workers whose threads have already terminated. `Exit()` → `Sleep()` → `_stopEvent.WaitOne()` then blocks forever (a dead thread never sets the event). The first collection's tests run; the second collection's fixture deadlocks in its constructor; the host eventually gets killed.

This is why single-collection (filtered) runs were fine — only one fixture ever bootstraps — but the full suite hangs. It's not a parallelization race: even strictly sequential, the second fixture deadlocks.

## Fix

**(a) Engine — idempotent `SerializationThreadWorker.Exit()`**
A second `Exit()` is now a safe no-op instead of a permanent block. Only the owning (main) thread calls `Exit()`, so no synchronization is needed, and the single-call production shutdown path is unchanged.

**(b) Tests — one shared bootstrap, strictly sequential collections**
- New `TestServerBootstrap.EnsureInitialized()` runs the superset global init **exactly once per process** (lock + once-flag).
- `UOContentFixture` / `PathfindingTestFixture` slim down to delegate to it and no longer tear down global state (which the single-bootstrap model owns for the host's lifetime).
- `[assembly: CollectionBehavior(DisableTestParallelization = true)]` so collections never overlap.

## Result

| | Before | After |
|---|---|---|
| Tests run (full suite) | 258 (UOContent collection deadlocked) | **418** |
| Outcome | 2.5-min hang → host crash | **418 passed, clean exit** |
| Wall time | killed | **~7s** |

Projects/UOContent.Tests/Fixtures/UOContentFixture.cs

@@ -1,63 +1,13 @@
-using System;
-using System.Reflection;
-using Server.Items;
-using Server.Misc;
-using Server.Tests.Maps;
 using Xunit;
 
 namespace Server.Tests;
 
 [CollectionDefinition("Sequential UOContent Tests", DisableParallelization = true)]
-public class UOContentFixture : ICollectionFixture<UOContentFixture>, IDisposable
+public class UOContentFixture : ICollectionFixture<UOContentFixture>
 {
-    public UOContentFixture()
-    {
-        Core.ApplicationAssembly = Assembly.GetExecutingAssembly();
-        Core.LoopContext = new EventLoopContext();
-        Core.Expansion = Expansion.EJ;
-
-        // Load Configurations
-        ServerConfiguration.Load(true);
-
-        // Load UOContent.dll into the type resolver
-        ServerConfiguration.AssemblyDirectories.Add(Core.BaseDirectory);
-        AssemblyHandler.LoadAssemblies(["Server.dll", "UOContent.dll"]);
-
-        // Load Skills
-        SkillsInfo.Configure();
-
-        // Configure networking (initializes RingSocketManager for tests)
-        Server.Network.NetState.Configure();
-
-        // Configure / Initialize
-        TestMapDefinitions.ConfigureTestMapDefinitions();
-
-        // Configure the world
-        World.Configure();
-
-        Timer.Init(0);
-
-        // Configure Races
-        RaceDefinitions.Configure();
-
-        // Load the world
-        World.Load();
-
-        World.ExitSerializationThreads();
-
-        DecayScheduler.Configure();
-    }
-
-    private static int _counter;
-
-    public void Dispose()
-    {
-        _counter++;
-
-        if (_counter > 1)
-        {
-            throw new Exception("NO!");
-        }
-        Timer.Init(0);
-    }
+    // All process-global initialization lives in TestServerInitializer and runs exactly once,
+    // shared across every collection fixture. Tearing down global state here is intentionally
+    // omitted: the world/serialization workers are initialized once and reused for the whole
+    // test host, so there is nothing per-collection to dispose.
+    public UOContentFixture() => TestServerInitializer.Initialize();
 }