refactor: use per-test StateBag instead of shared static state

Replace the shared static List<string> with per-test context StateBag
storage. Each test gets its own isolated list, eliminating the race
condition at the root rather than working around it with [NotInParallel]
and type guards.

The type guard in the BeforeEvery hook is still needed (it fires for
all tests) but no longer protects shared mutable state.

TUnit.Engine.Tests/HookExecutionOrderTests.cs

@@ -13,31 +13,30 @@ public static void GlobalSetup(TestContext context)
             return;
         }
 
-        HookExecutionOrderTests._executionOrder.Clear();
-        HookExecutionOrderTests._executionOrder.Add("BeforeEvery");
+        var order = context.StateBag.GetOrAdd<List<string>>("executionOrder", _ => []);
+        order.Add("BeforeEvery");
     }
 }
 
-[NotInParallel]
 public class HookExecutionOrderTests
 {
-    internal static readonly List<string> _executionOrder = [];
-
     [Before(Test)]
     public void InstanceSetup()
     {
-        _executionOrder.Add("Before");
+        var order = TestContext.Current!.StateBag.GetOrAdd<List<string>>("executionOrder", _ => []);
+        order.Add("Before");
     }
 
     [Test]
     public void VerifyExecutionOrder()
     {
-        _executionOrder.Add("Test");
+        var order = TestContext.Current!.StateBag.GetOrAdd<List<string>>("executionOrder", _ => []);
+        order.Add("Test");
 
         // Verify that BeforeEvery runs before Before
-        _executionOrder.Count.ShouldBe(3);
-        _executionOrder[0].ShouldBe("BeforeEvery");
-        _executionOrder[1].ShouldBe("Before");
-        _executionOrder[2].ShouldBe("Test");
+        order.Count.ShouldBe(3);
+        order[0].ShouldBe("BeforeEvery");
+        order[1].ShouldBe("Before");
+        order[2].ShouldBe("Test");
     }
 }