Adding OpenAI agents sample

It would be great to add a sample based on OpenAI’s [agents](https://github.com/openai/openai-agents-python) framework.

Look at existing samples in the repository to understand the structure and how they integrate the A2A Protocol. Review the OpenAI agents framework documentation and examples. The goal is to create a new Jupyter Notebook sample that demonstrates using OpenAI agents within the A2A context. Check for any existing agent-related code or discussions in the repo.

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