Fix race condition when cancelling pending HTTP connection attempts

[MihaZupan]

Fixes #110598

There's a race condition here where if the initiating request completes right away, before we're able to set the CTS in InjectNewHttp11ConnectionAsync, we'll never set the timeout.

runtime/src/libraries/System.Net.Http/src/System/Net/Http/SocketsHttpHandler/ConnectionPool/HttpConnectionWaiter.cs

Lines 78 to 93 in c969265

	public void CancelIfNecessary(HttpConnectionPool pool, bool requestCancelled)
	{
	int timeout = GlobalHttpSettings.SocketsHttpHandler.PendingConnectionTimeoutOnRequestCompletion;
	if (ConnectionCancellationTokenSource is null ||
	timeout == Timeout.Infinite ||
	pool.Settings._connectTimeout != Timeout.InfiniteTimeSpan && timeout > (int)pool.Settings._connectTimeout.TotalMilliseconds) // Do not override shorter ConnectTimeout
	{
	return;
	}
	lock (this)
	{
	if (ConnectionCancellationTokenSource is null)
	{
	return;
	}

runtime/src/libraries/System.Net.Http/src/System/Net/Http/SocketsHttpHandler/ConnectionPool/HttpConnectionPool.Http1.cs

Lines 256 to 263 in c969265

	await Task.CompletedTask.ConfigureAwait(ConfigureAwaitOptions.ForceYielding);
	HttpConnectionWaiter<HttpConnection> waiter = queueItem.Waiter;
	HttpConnection? connection = null;
	Exception? connectionException = null;
	CancellationTokenSource cts = GetConnectTimeoutCancellationTokenSource();
	waiter.ConnectionCancellationTokenSource = cts;

It's easy to reproduce if you add a delay between the Yield and assigning the cts, while failing the request.
The test I added was 100% deterministic if I added the delay, or increased the Parallel count from 100 to 1000, but that takes too long to run for CI.

The HTTP/3 change is just adding a try/finally to signal the waiter, looks like we missed that in #101531.
Diff without whitespace changes

[MihaZupan]

/azp run runtime-libraries-coreclr outerloop

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[MihaZupan]

/backport to release/9.0-staging

[github-actions]

Started backporting to release/9.0-staging: https://github.com/dotnet/runtime/actions/runs/12364986878

[MihaZupan]

With HTTP 1 and 2, we do this here instead:

runtime/src/libraries/System.Net.Http/src/System/Net/Http/SocketsHttpHandler/ConnectionPool/HttpConnectionPool.cs

Lines 562 to 563 in ce9dd2a

	http11ConnectionWaiter?.CancelIfNecessary(this, cancellationToken.IsCancellationRequested);
	http2ConnectionWaiter?.CancelIfNecessary(this, cancellationToken.IsCancellationRequested);

[antonfirsov]

LGTM modulo nitpicking comments above.