context.signal_entity causes sub orchestrator to never finish.

🐛 **Describe the bug**
Using `context.signal_entity` causes a sub-orchestrator to never finish. That is, it prevents `StopIteration` from being raised.

🤔 **Expected behavior**
Signalling an entity does not cause the generator to fail

☕ **Steps to reproduce**
Create an entity and signal it from within a sub orchestrator. This will cause it to not finish. Now switch it to use `context.call_entity`, which works as expected.

首先复现所描述的最小 entity 和 sub-orchestrator 案例，然后比较 context.signal_entity 和 context.call_entity 这两个入口点，同时跟踪 StopIteration 如何完成 generator。完成的标准是：signal entity 不再阻止 sub-orchestrator 完成，并为预期行为提供回归覆盖。

由索引模型根据 Issue 内容生成。