Pull request overview

This PR fixes a race condition in the Send_TimeoutRequestContent_Throws test where the server connection could close before the client properly timed out, causing the test to fail intermittently with an HttpRequestException instead of the expected TaskCanceledException with TimeoutException inner exception.

Changes:

• Removed the [ActiveIssue] attribute that was tracking the flaky test
• Added semaphore-based synchronization between client and server to ensure the server stays alive until the client completes its assertions

Thanks.

I wonder if we should make this an option on the factory eventually, e.g. DelayServerShutdownUntilClientCompletion. Not sure it's feasible/worth it to wire though though.
We have such SemaphoreSlim/TaskCompletionSource in a bunch of places.