feat: add AgentAsTool

[notowen333]

Description

Will close #1002.

This PR adds the AgentAsTool class as well as a convenience method on the Agent class to return an instance of it.

The AgentAsTool class just calls stream_async on the underlying agent and formats the response as a ToolResultEvent.

The added integration test confirms that an AgentAsTool was invoked and that agent-as-tool used a tool of its own.

Related Issues

#1002

Documentation PR

Not yet provided. I was planning to follow up with this.

Type of Change

New feature

Testing

• Added unit tests

• Added a minimal integ test

• [ x] I ran hatch run prepare

Checklist

• [ x ] I have read the CONTRIBUTING document
• [ x ] I have added any necessary tests that prove my fix is effective or my feature works
• I have updated the documentation accordingly
• [ x ] I have added an appropriate example to the documentation to outline the feature, or no new docs are needed
• [ x ] My changes generate no new warnings
• [ x ] Any dependent changes have been merged and published

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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