test: deflake realtime sideband close

[dylan-hurd-oai]

Summary

• Keep the mocked WebRTC sideband accept delayed beyond the negative-event observation window.
• Removes the scheduler race where slow CI could complete the mock handshake before close aborts the sideband task.

Verification

• just fmt
• cargo test -p codex-core --test all suite::realtime_conversation::conversation_webrtc_close_while_sideband_connecting_drops_pending_join -- --exact (3 consecutive runs)