Fix test Http3_WaitForConnection_RecordedWhenWaitingForStream

[antonfirsov]

#114083 incorrectly added a test that uses ActivityRecorder without process isolation via RemoteExecutor.

This may lead to the registered ActivityListener callbacks firing from unrelated tests running in parallel. I started seeing frequent local failures because of this, the stack trace in #116279 (comment) also seems to be related.

Fixes #116279.

[Copilot]

Pull Request Overview

This PR fixes an issue with a test that uses ActivityRecorder without proper process isolation, preventing interference from parallel tests. The key changes include:

• Marking CreateHttp3LoopbackServer as static in HttpClientHandlerTestBase.SocketsHttpHandler.cs.
• Updating DiagnosticsTests.cs to require RemoteExecutor support and wrapping the test logic within RemoteExecutor.Invoke.

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/libraries/System.Net.Http/tests/FunctionalTests/HttpClientHandlerTestBase.SocketsHttpHandler.cs	Made CreateHttp3LoopbackServer static for improved clarity and test isolation.
src/libraries/System.Net.Http/tests/FunctionalTests/DiagnosticsTests.cs	Updated the test condition to use RemoteExecutor and refactored test logic into a static local function for isolation.

Comments suppressed due to low confidence (2)

src/libraries/System.Net.Http/tests/FunctionalTests/HttpClientHandlerTestBase.SocketsHttpHandler.cs:66

• Marking CreateHttp3LoopbackServer as static is appropriate since it doesn't depend on instance state. Please verify that no existing code depends on an instance method signature.

        protected static Http3LoopbackServer CreateHttp3LoopbackServer(Http3Options options = default)

src/libraries/System.Net.Http/tests/FunctionalTests/DiagnosticsTests.cs:1594

• Using RemoteExecutor to wrap the test ensures isolation from other parallel runs. Confirm that the test environment supports RemoteExecutor for consistent behavior.

        [ConditionalFact(typeof(RemoteExecutor), nameof(RemoteExecutor.IsSupported))]

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[rokonec]

It is same root cause as in #116955.

However, during investigation I have noticed another two tests which are using ActivityRecorder:
SendAsync_ConnectionFailure_RecordsActivitiesWithCorrectErrorInfo
SendAsync_Success_ConnectionSetupActivityGraphRecorded

Maybe we should move those to RemoteExecutor as well.

[antonfirsov]

@rokonec those 2 do utilze RemoteExecutor already for isolation. Do you have more info on failures related to those tests?

I merged this now, if we have further ideas we can file them in separate PR-s.

Also note that #115683 has been reported before the merge of #114083, so I wouldn't exclude the option that there may exist alternative reasons for the timeouts, unrelated to ActivityRecorder. I would keep #115683 open for a while and monitor if the failures still keep happening.