fix(asyncio): Avoid InvalidStateError on late callbacks after cancel

[goodvibs]

Summary

Guard _set_future so scheduled completions are skipped when the asyncio Future is cancelled.

Problem

_set_future schedules complete() via call_soon_threadsafe. If the Future is cancelled, a late callback could still call set_result / set_exception, which raises asyncio.InvalidStateError and is logged as an exception in the event loop callback.

Solution

At the start of complete(), return early if future.done(). This matches common asyncio callback bridges (no-op when the future is no longer pending).

Testing

Added AsyncioSetFutureTest with two cases (no live broker): cancelled Future + late success callback, and already-resolved Future + late callback.

[goodvibs]

Same as #296, but from a different branch (the other one was off my main branch, which I needed to add commits to)