make Http2_RequestFailsWithAppropriateHttpProtocolException test more deterministic

[wfurt]

When I look at the normal passing run I see something like

when it fails, the exchange looks like

So it seems like if the GOAWAY is sent before the client even start making request the test fails with Couldn't find HttpProtocolException with matching error code in exception: System.Net.Http.HttpRequestException: An HTTP/2 connection could not be established because the server did not complete the HTTP/2 handshake.

This may (or may not) be caused by the connection pool refactor. Since the requests are now detached, the connection can be closed by the 'GOAWAY` and the request does not fail with protocol error.

To minimize risk, I only changed the test to wait for beginning of the request to arrive so when HttpClient process the response it would produce deterministic result. I was running tests in loop with this change and I did not see failure. It was failing for me before frequently with less than 20-50 iterations.

fixes #92647

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Issue Details

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When I look at the normal passing run I see something like

when it fails, the exchange looks like

So it seems like if the GOAWAY is sent before the client even start making request the test fails with Couldn't find HttpProtocolException with matching error code in exception: System.Net.Http.HttpRequestException: An HTTP/2 connection could not be established because the server did not complete the HTTP/2 handshake.

This may (or may not) be caused by the connection pool refactor. Since the requests are now detached, the connection can be closed by the 'GOAWAY` and the request does not fail with protocol error.

To minimize risk, I only changed the test to wait for beginning of the request to arrive so when HttpClient process the response it would produce deterministic result. I was running tests in loop with this change and I did not see failure. It was failing for me before frequently with less than 20-50 iterations.

fixes #92647

Author:	wfurt
Assignees:	wfurt
Labels:	area-System.Net.Http, test-enhancement
Milestone:	9.0.0

[antonfirsov]

LGTM

[wfurt]

enterprised-linux failures are unrelated. looks like infrastructure problem.

[ManickaP]

/backport to release/8.0-staging

[github-actions]

Started backporting to release/8.0-staging: https://github.com/dotnet/runtime/actions/runs/17242678894