Fix flaky MaxResponseHeadersLength tests by handling additional SocketError codes

[Copilot]

Description

The ThresholdExceeded_ThrowsException and LargeSingleHeader_ThrowsException tests were flaking when the client forcibly closed the connection after detecting excessive response headers, but before the server finished writing. The server-side loopback only handled SocketError.Shutdown, missing ConnectionReset and ConnectionAborted which manifest on different platforms/timings for the same race condition.

Changes

Extended exception filters in both test methods to catch all three expected SocketError codes:

// Before
catch (IOException ex) when (ex.InnerException is SocketException se && se.SocketErrorCode == SocketError.Shutdown) { }

// After  
catch (IOException ex) when (ex.InnerException is SocketException se && (se.SocketErrorCode == SocketError.Shutdown || se.SocketErrorCode == SocketError.ConnectionReset || se.SocketErrorCode == SocketError.ConnectionAborted)) { }

This aligns with the existing pattern in LoopbackProxyServer.cs which already handles both ConnectionReset and ConnectionAborted together.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>[HTTP] ThresholdExceeded_ThrowsException</issue_title>
<issue_description>## Build Information
Build: https://dev.azure.com/dnceng-public/cbb18261-c48f-4abb-8651-8cdcb5474649/_build/results?buildId=1296343
Build error leg or test failing: System.Net.Http.Functional.Tests.WorkItemExecution
Pull request: #124469

Error Message

Fill the error message using step by step known issues guidance.

{
  "ErrorMessage": "",
  "ErrorPattern": "ThresholdExceeded_ThrowsException.*\[FAIL\]",
  "BuildRetry": false,
  "ExcludeConsoleLog": false
}

</issue_description>

<agent_instructions>The exception thrown is

System.IO.IOException : Unable to write data to the transport connection: An existing connection was forcibly closed by the remote host..
---- System.Net.Sockets.SocketException : An existing connection was forcibly closed by the remote host.
   at System.Net.Test.Common.LoopbackServer.Connection.SendResponseAsync(Byte[] response) in /_/src/libraries/Common/tests/System/Net/Http/LoopbackServer.cs:line 762
   at System.Net.Test.Common.LoopbackServer.Connection.SendResponseBodyAsync(Byte[] content, Boolean isFinal) in /_/src/libraries/Common/tests/System/Net/Http/LoopbackServer.cs:line 1018
   at System.Net.Test.Common.GenericLoopbackConnection.SendResponseBodyAsync(String content, Boolean isFinal) in /_/src/libraries/Common/tests/System/Net/Http/GenericLoopbackServer.cs:line 172
   at System.Net.Test.Common.LoopbackServer.Connection.SendResponseAsync(HttpStatusCode statusCode, IList`1 headers, String content, Boolean isFinal) in /_/src/libraries/Common/tests/System/Net/Http/LoopbackServer.cs:line 966
   at System.Net.Http.Functional.Tests.HttpClientHandler_MaxResponseHeadersLength_Test.<>c__DisplayClass6_0.<ThresholdExceeded_ThrowsException>b__1(GenericLoopbackServer server)

Fix the test to account for this being a possibility</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes [HTTP] ThresholdExceeded_ThrowsException #124515

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[ManickaP]

LGTM

[Copilot]

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