feat(agent-interface): introduce AgentBase Protocol as the interface for agent classes to implement

[awsarron]

Description

Adds an AgentBase Protocol that defines a minimal interface for agent classes.

This is in part to support #907 where we will introduce an A2AAgent class alongside the current Agent class. Both Agent and A2AAgent need the same functions and signatures for our multiagent classes like Graph to consume - AgentBase is that consistent interface.

The purpose of the AgentBase interface is to define the signatures for methods that are used in higher level primitives, like multiagent orchestrators (e.g. Graph).

Related Issues

#573

Documentation PR

TODO

Type of Change

New feature

Testing

How have you tested the change? Verify that the changes do not break functionality or introduce warnings in consuming repositories: agents-docs, agents-tools, agents-cli

• I ran hatch run prepare

Checklist

• I have read the CONTRIBUTING document
• [N/A] I have added any necessary tests that prove my fix is effective or my feature works
• [TODO] I have updated the documentation accordingly
• [TODO] I have added an appropriate example to the documentation to outline the feature, or no new docs are needed
• My changes generate no new warnings
• 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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[dbschmigelski]

🥳

Just calling out here if anyone looks back

• structured_output() is deprecated so it wasn't added
• Direct tool calling also was not added

[mkmeral]

nit: How would this relate to MultiAgentBase? I personally want to get rid of it, but given that AgentBase is more extensive than MultiAgentBase, it might not be possible, right?