[HTTP] Fix new header test

[ManickaP]

Use loopback server since some of the validation happens when serializing to the wire which is after establishing the connection.

Fixes #117198

Compare without whitespace.

[dotnet-policy-service]

Tagging subscribers to this area: @dotnet/ncl
See info in area-owners.md if you want to be subscribed.

[Copilot]

Pull Request Overview

Fix the new header test by sending the request over a loopback server so that header serialization happens on the wire.

• Wrap the existing test logic in LoopbackServerFactory.CreateClientAndServerAsync with separate client/server callbacks.
• Introduce a TaskCompletionSource<bool> to coordinate whether the server should accept the connection.
• Change CookieContainer.Add(new Uri(uri), …) to the overload CookieContainer.Add(uri, …).

Comments suppressed due to low confidence (3)

src/libraries/Common/tests/System/Net/Http/HttpClientHandlerTest.cs:2175

• Consider specifying TaskCreationOptions.RunContinuationsAsynchronously when creating the TaskCompletionSource to avoid potential deadlocks from synchronous continuations.

            TaskCompletionSource<bool> acceptConnection = new TaskCompletionSource<bool>();

src/libraries/Common/tests/System/Net/Http/HttpClientHandlerTest.cs:2205

• Use TrySetResult instead of SetResult to prevent an InvalidOperationException if the TaskCompletionSource is completed more than once.

                    acceptConnection.SetResult(false);

src/libraries/Common/tests/System/Net/Http/HttpClientHandlerTest.cs:2177

• Ensure the TaskCompletionSource is completed in all code paths (including unexpected exceptions) by adding a catch-all or finally block inside the client lambda, otherwise the server callback may hang waiting for the result.

            await LoopbackServerFactory.CreateClientAndServerAsync(async uri =>

[MihaZupan]

Thanks

[CarnaViire]

LGTM, thanks