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"")
```

Search for CRTResponseBody._on_complete and inspect how the output stream's future is completed during close. Reproduce the BiDi API output-stream close path, then verify that closing it no longer raises InvalidStateError when the future is cancelled.

Vom Indexierungsmodell aus dem Issue-Text verfasst.