[Task]: Remove hard-code text prompt in multi-turn path

[akshaykalkunte]

📝 Description

We add a hard-coded prompt "Please respond to the audio" in the multi-turn path for tasks with audio. Let's move this to the task configs instead.

🔍 Context

Hard-coding the prompt results in unexpected behavior, reduced interpretability and flexibility.

✅ Acceptance Criteria

• Multi-turn audio benchmarks like MT-Bench Audio work as expected and scores match from before.

📦 Related Work

🙌 Additional Notes