test(state): 更新状态机上下文获取测试验证逻辑

- 将断言修改为验证 GetContext 方法抛出 InvalidOperationException 异常
- 在状态机初始化前验证状态上下文为空的预期行为
- 保持初始化后状态上下文正确设置的验证逻辑

GFramework.Core.Tests/state/StateMachineSystemTests.cs

@@ -111,8 +111,8 @@ public class StateMachineSystemTests
         _stateMachine!.Register(state1);
         _stateMachine.Register(state2);
 
-        Assert.That(state1.GetContext(), Is.Null);
+        Assert.Throws<InvalidOperationException>(() => state1.GetContext());
-        Assert.That(state2.GetContext(), Is.Null);
+        Assert.Throws<InvalidOperationException>(() => state2.GetContext());
 
         _stateMachine.Init();