feat(core): allow skip_reply when calling commit_user_turn

[chenghao-mou]

Expose skip_reply at session level for both realtime and cascading agents when calling commit_user_turn.

closes #5026

This can be tested with a RPC:

    @ctx.room.local_participant.register_rpc_method("commit_user_turn")
    async def on_commit_user_turn(data: rtc.RpcInvocationData) -> str:
        skip_reply = data.payload == "skip_reply"
        logger.info(f"RPC commit_user_turn called, skip_reply={skip_reply}")
        fut = session.commit_user_turn(skip_reply=skip_reply)
        transcript = await fut
        logger.info(f"Committed transcript: {transcript}")
        return transcript

[devin-ai-integration]

✅ Devin Review: No Issues Found

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