Fix MTP TaskExtensions

[Youssef1313]

FYI @SimonCropp

I also think the whole implementation here is not correct. It feels to me like if cancellation was requested in the middle of the original task, it won't immediately return a completed task.

Originally posted by @Youssef1313 in https://github.com/microsoft/testfx/pull/6861#discussion_r2485042452

[Youssef1313]

Opening an issue to track this. We also would like to have a test on our side that captures the bug.

[SimonCropp]

@Youssef1313 @Evangelink can you both review this https://github.com/SimonCropp/Polyfill/pull/410

[Evangelink]

@SimonCropp fix is in 9.0.3, right?

[Youssef1313]

I think yes. I'd still like us to add a test though.