OpenAI: Use structured-output-as-a-tool instead of `output_type`

[njbrake]

Using openai/openai-agents-python#203, we can adjust the behavior of our agent such that it must call tools and allows us to pass in a "final output" type tool.

Let's refactor our agent to follow this design to be more consistent with the design in other agents.

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[github-actions]

This issue is stale because it has been open 30 days with no activity. Remove stale label or comment or this will be closed in 7 days.

[github-actions]

This issue was closed because it has been stalled for 7 days with no activity.