Better handling of failed to summarize failure reason

[wintonzheng]

[!IMPORTANT] Improve logging and error handling in summary_failure_reason_for_max_steps() in agent.py.

• Logging:
  • Change log level from exception to warning in summary_failure_reason_for_max_steps() in agent.py.
  • Add task_id and step_id to log message for better traceability.
• Error Handling:
  • Add fallback return value in summary_failure_reason_for_max_steps() to return last step's action results if an exception occurs.

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