Fix instructions not being applied

[michieldwitte]

        runner = RealtimeRunner(
            starting_agent=RealtimeAgent(
                name="realtime agent",
                tools=tools,
            )
        )
        model_config = {"initial_model_settings": {"instructions": "Specific instructions"}}
        await runner.run(model_config = model_cfg)

I would assume we first create the runner with an agent.
And then when actually running we can add specific instructions.
Currently these instructions are ignored.