3/4 Remove local realtime playback interruption

[aibrahim-oai]

Stack position: 3/4

Stack:

• 1/4 1/4 Replace realtime websocket transport with WebRTC #16805 Replace realtime websocket transport with WebRTC
• 2/4 2/4 Route TUI realtime audio through shared echo cancellation #16806 Route TUI realtime audio through shared echo cancellation
• 3/4 3/4 Remove local realtime playback interruption #16807 Remove local realtime playback interruption (this PR)
• 4/4 4/4 Support ChatGPT realtime call auth and request shape #16769 Support ChatGPT realtime call auth and request shape

This PR:

• Stop locally clearing speaker playback on input speech-start events.
• Keep interruption behavior owned by the transport/session side instead of resetting the local queue in the TUI.