[H/3] Fix response header tests for H/3

[ManickaP]

This time server was the one closing the connection while the client is still waiting on stream abort:

System.Net.Http.Functional.Tests.SocketsHttpHandler_HttpClientHandler_MaxResponseHeadersLength_Http3.ThresholdExceeded_ThrowsException(maxResponseHeadersLength: 2684354, headersLengthEstimate: 102400) [FAIL]
      System.Net.Http.HttpRequestException : An error occurred while sending the request.
      ---- System.Net.Http.HttpProtocolException : The HTTP/3 server closed the connection. HTTP/3 error code 'H3_NO_ERROR' (0x100). (HttpProtocolError)
      Stack Trace:
        /_/src/libraries/System.Net.Http/src/System/Net/Http/SocketsHttpHandler/Http3RequestStream.cs(321,0): at System.Net.Http.Http3RequestStream.SendAsync(CancellationToken cancellationToken)
        /_/src/libraries/System.Net.Http/src/System/Net/Http/SocketsHttpHandler/Http3RequestStream.cs(398,0): at System.Net.Http.Http3RequestStream.SendAsync(CancellationToken cancellationToken)
        /_/src/libraries/System.Net.Http/src/System/Net/Http/SocketsHttpHandler/Http3Connection.cs(339,0): at System.Net.Http.Http3Connection.SendAsync(HttpRequestMessage request, WaitForHttp3ConnectionActivity waitForConnectionActivity, Boolean streamAvailable, CancellationToken cancellationToken)
        /_/src/libraries/System.Net.Http/src/System/Net/Http/SocketsHttpHandler/Http3Connection.cs(353,0): at System.Net.Http.Http3Connection.SendAsync(HttpRequestMessage request, WaitForHttp3ConnectionActivity waitForConnectionActivity, Boolean streamAvailable, CancellationToken cancellationToken)
        /_/src/libraries/System.Net.Http/src/System/Net/Http/SocketsHttpHandler/ConnectionPool/HttpConnectionPool.Http3.cs(107,0): at System.Net.Http.HttpConnectionPool.TrySendUsingHttp3Async(HttpRequestMessage request, CancellationToken cancellationToken)
        /_/src/libraries/System.Net.Http/src/System/Net/Http/SocketsHttpHandler/ConnectionPool/HttpConnectionPool.cs(419,0): at System.Net.Http.HttpConnectionPool.SendWithVersionDetectionAndRetryAsync(HttpRequestMessage request, Boolean async, Boolean doRequestAuth, CancellationToken cancellationToken)
        /_/src/libraries/System.Net.Http/src/System/Net/Http/SocketsHttpHandler/RedirectHandler.cs(29,0): at System.Net.Http.RedirectHandler.SendAsync(HttpRequestMessage request, Boolean async, CancellationToken cancellationToken)
        /_/src/libraries/System.Net.Http/src/System/Net/Http/SocketsHttpHandler/SocketsHttpHandler.cs(629,0): at System.Net.Http.SocketsHttpHandler.<SendAsync>g__CreateHandlerAndSendAsync|115_0(HttpRequestMessage request, CancellationToken cancellationToken)
        /_/src/libraries/System.Net.Http/src/System/Net/Http/HttpClient.cs(523,0): at System.Net.Http.HttpClient.<SendAsync>g__Core|83_0(HttpRequestMessage request, HttpCompletionOption completionOption, CancellationTokenSource cts, Boolean disposeCts, CancellationTokenSource pendingRequestsCts, CancellationToken originalCancellationToken)
        /_/src/libraries/Common/tests/System/Net/Http/HttpClientHandlerTest.MaxResponseHeadersLength.cs(136,0): at System.Net.Http.Functional.Tests.HttpClientHandler_MaxResponseHeadersLength_Test.<>c__DisplayClass6_0.<<ThresholdExceeded_ThrowsException>b__0>d.MoveNext()
        --- End of stack trace from previous location ---
        /_/src/libraries/Common/tests/System/Threading/Tasks/TaskTimeoutExtensions.cs(120,0): at System.Threading.Tasks.TaskTimeoutExtensions.GetRealException(Task task)
        --- End of stack trace from previous location ---
        /_/src/libraries/Common/tests/System/Threading/Tasks/TaskTimeoutExtensions.cs(90,0): at System.Threading.Tasks.TaskTimeoutExtensions.WhenAllOrAnyFailed(Task[] tasks)
        /_/src/libraries/Common/tests/System/Net/Http/GenericLoopbackServer.cs(44,0): at System.Net.Test.Common.LoopbackServerFactory.<>c__DisplayClass6_0.<<CreateClientAndServerAsync>b__0>d.MoveNext()
        --- End of stack trace from previous location ---
        /_/src/libraries/Common/tests/System/Net/Http/Http3LoopbackServer.cs(115,0): at System.Net.Test.Common.Http3LoopbackServerFactory.CreateServerAsync(Func`3 funcAsync, Int32 millisecondsTimeout, GenericLoopbackOptions options)
        /_/src/libraries/Common/tests/System/Net/Http/HttpClientHandlerTest.MaxResponseHeadersLength.cs(123,0): at System.Net.Http.Functional.Tests.HttpClientHandler_MaxResponseHeadersLength_Test.ThresholdExceeded_ThrowsException(Nullable`1 maxResponseHeadersLength, Int32 headersLengthEstimate)
        --- End of stack trace from previous location ---
        ----- Inner Stack Trace -----
        

So wait with connection disposal on both sides.

Fixes #117484

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Pull Request Overview

This PR updates two HTTP/3 header length tests to synchronize client and server shutdowns using CountdownEvent instead of SemaphoreSlim, preventing premature connection closure and test flakiness.

• Replace SemaphoreSlim with CountdownEvent(2) for two-party coordination.
• Signal and wait on the countdown event in both client and server callbacks.
• Apply a timeout to the wait to avoid indefinite blocking.

Comments suppressed due to low confidence (1)

src/libraries/Common/tests/System/Net/Http/HttpClientHandlerTest.MaxResponseHeadersLength.cs:76

• [nitpick] The variable name ce is abbreviated and may be unclear to readers. Consider renaming it to countdownEvent for clarity.

            var ce = new CountdownEvent(2);

[ManickaP]

/azp run runtime

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