Stabilize websocket response.failed error delivery

[aibrahim-oai]

What changed

• Drop failed websocket connections immediately after a terminal stream error instead of awaiting a graceful close handshake before forwarding the error to the caller.
• Keep the success path and the closed-connection guard behavior unchanged.

Why this fixes the flake

• The failing integration test waits for the second websocket stream to surface the model error before issuing a follow-up request.
• On slower runners, the old error path awaited ws_stream.close().await before sending the error downstream. If that close handshake stalled, the test kept waiting for an error that had already happened server-side and nextest timed it out.
• Dropping the failed websocket immediately makes the terminal error observable right away and marks the session closed so the next request reconnects cleanly instead of depending on a best-effort close handshake.

Code or test?

• This is a production logic fix in codex-api. The existing websocket integration test already exercises the regression path.