Add support for registering Pydantic-AI agents as OpenAI-compatible endpoints

### Description


Registry that allows Pydantic-AI agents to be registered and exposed via a FastAPI OpenAI-compatible endpoint with routes like: 

- `/v1/chat/completions`
- `/v1/responses`
- `/v1/models`



### References

Inspired from: https://github.com/pydantic/pydantic-ai/pull/2041