InvalidStateError on output stream close in BiDi API

[BumaldaOverTheWater94]

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"")