InvalidStateError on output stream close in BiDi API

When using the BiDi API and closing the output stream, I always see low-level CRT futures that raise `concurrent.futures.InvalidStateError`

This seems to be occurring because `CRTResponseBody._on_complete()` is invoking `future.set_result(b"")` when the future is already in a `CANCELLED` state.

Although the error is mostly cosmetic, it still interrupts any higher-level async task loops.

Please add a guard or something like:
```
if not future.cancelled():
future.set_results(b"")
```

搜索 CRTResponseBody._on_complete，并检查输出流的 future 在关闭期间是如何完成的。复现 BiDi API 输出流的关闭路径，然后验证在 future 被取消时，关闭它不再引发 InvalidStateError。

由索引模型根据 Issue 内容生成。