RFR: 8372147: ConnectionFlowControlTest should use HttpResponse.connectionLabel()

[Volkan Yazici]

On Thu, 27 Nov 2025 13:03:28 GMT, Volkan Yazici <vyazici at openjdk.org> wrote:

> Use `HttpResponse::connectionLabel` instead of `Http2TestExchange::connectionKey` to identify the connection request-response exchange is performed on, for the latter can accidentally return the same identifier again, e.g., if a closed connection is immediately reused.

Run the test 100 times on several platforms – results are green.

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PR Comment: https://git.openjdk.org/jdk/pull/28534#issuecomment-3589065660