Fix Windows IPC race condition when using parallel checking

[JukkaL]

Cancellation is asynchronous, so some data could be read after calling cancel(). Rework ready_to_read to first cancel all operations and then check for each if data is available.

See discussion in #21221 for more context.

I heavily relied on coding agent assist for this, but I did multiple review iterations and refinements.

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅

[ilevkivskyi]

Thanks! Yeah, let's try this.

cc @emmatyping (just FYI, feel free to ignore if you don't have time)

[ilevkivskyi]

TBH I don't know if this can raise itself. Also maybe we can combine two loops, i.e.:

for i, ov in pending:
    try:
        ov.cancel()
        _, err = ov.GetOverlappedResult(True)
    except OSError as e:
        ...
    ...

(but to be clear I know very little about Windows)

[JukkaL]

Claude Code was confident that this won't raise, but Codex warned about this potentially raising. I could look into the Python stdlib to see if they expect exceptions in similar calls.

[JukkaL]

CPython has two loops for a similar use case, and it also appears to assume that cancel() doesn't raise in a similar use case.