Cancel OkHttp WebSocket when connect is canceled

[masaru87]

Summary

• close the receive channel when connect is canceled during the WebSocket handshake
• let the existing channel close handler cancel the underlying OkHttp WebSocket
• preserve and rethrow the original CancellationException
• add a regression test for cancellation while opening

Previously, canceling the coroutine suspended in connect stopped only the caller. The OkHttp WebSocket connection attempt remained active because neither the receive channel nor the socket was canceled.

Testing

• ./gradlew :engine:okhttp:test --no-daemon
• ./gradlew jvmTest --no-daemon