feat: handle missing final_info.json after successful experiment runs

[chun61205]

While using run_experiment, I discovered that even if the experiment completes successfully, the script will crash if the output file is not named final_info.json. This happens because the postprocessing logic assumes the presence of this file:

AI-Scientist/ai_scientist/perform_experiments.py

Lines 61 to 66 in 9dffdc4

	else:
	with open(osp.join(cwd, f"run_{run_num}", "final_info.json"), "r") as f:
	results = json.load(f)
	results = {k: v["means"] for k, v in results.items()}
	next_prompt = f"""Run {run_num} completed. Here are the results:

Therefore, this pr

1. Adds an explicit check for the existence of final_info.json after a successful experiment run.
2. Adds an extra prompt signal to remind the model (or agent) that experiment.py must generate final_info.json.