fix(realtime): scope delayed audio guardrail interruption

[seratch]

This pull request fixes delayed Realtime audio output guardrails so they interrupt only the buffered playback produced by their source response, without cancelling or clearing a newer response.

#3933 originally combined three distinct Realtime changes:

1. Apply output guardrails to text deltas.
2. Synchronize after-turn streaming cancellation with event consumption.
3. Make delayed audio guardrail interruption response-scoped and bound the associated audio tracking lifecycle.

That pull request was closed so the work could be reviewed and landed as three focused changes. #4124 delivered text-output guardrails, and #4130 delivered after-turn streaming cancellation. This pull request completes the third and final part.

The implementation:

• associates received audio and pending guardrail tasks with their source response;
• uses response-scoped, playback-only interruption when an audio guardrail finishes after its source response has ended;
• preserves newer response playback across asynchronous interruption listeners;
• suppresses late audio deltas from an interrupted source response;
• retains response audio indexes until guardrail work and local playback have settled;
• releases timers, listeners, tombstones, and response indexes during terminal cleanup;
• preserves existing custom RealtimeModel construction and positional dataclass behavior; and
• documents that custom transports must honor response_id and playback_only to provide the same source-scoped behavior.

[chatgpt-codex-connector]

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