fix: openapi working in sub-app

[racinmat]

The A2AFastAPIApplication is very useful, but currently can not be used as a Sub Application without breaking the openapi specification.
Lifespan does not work for Sub Applications, which makes this otherwise useful integration work in existing larger FastAPI apps.

This fix gets rid of the lifespan and instead enriches the openapi on the first call of the openapi method.

I tested it locally and everything works with sub-application.

The following works after the fix with openapi components populated:

from fastapi import FastAPI
from a2a.server.apps import A2AFastAPIApplication
app = FastAPI()
agent_app = A2AFastAPIApplication(...).build()
app.mount("/a2a", agent_app)

I am not adding @override although this overrides the openapi, because the pyright fails on it, see the failed lint run.

[gemini-code-assist]

Summary of Changes

Hello @racinmat, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses a bug where the OpenAPI schema for A2A-specific components was not being correctly generated, particularly when the FastAPI application was operating as a sub-application. The core solution involves refactoring the custom OpenAPI component injection mechanism from a lifespan event to an overridden openapi() method within a dedicated FastAPI subclass, ensuring consistent and correct schema generation.

Highlights

• Custom FastAPI Subclass: I've introduced A2AFastAPI, a custom subclass of FastAPI, specifically designed to encapsulate and manage the addition of A2A-specific OpenAPI schema components.
• Refactored OpenAPI Schema Generation: The logic for injecting A2ARequest and related schemas into the OpenAPI definition has been moved. Previously handled within a lifespan event handler, this is now managed by overriding the openapi() method within the new A2AFastAPI class. This change ensures the schema is correctly built, especially when the application is mounted as a sub-application, resolving the reported issue.
• Python Version Compatibility: To ensure broader compatibility, I've added conditional imports for the override decorator, using typing_extensions.override for Python versions prior to 3.12 and typing.override for Python 3.12 and later.

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[gemini-code-assist]

Code Review

This pull request refactors the OpenAPI generation for A2A sub-applications by introducing the A2AFastAPI class. The changes improve the handling of OpenAPI schema generation in sub-applications.

[holtskinner]

@racinmat Thanks for the contribution, can you add tests for your change to show that it works with sub-applications?

[racinmat]

@holtskinner added

[holtskinner]

@kthota-g @pstephengoogle Can you please review?

[racinmat]

@kthota-g @pstephengoogle please, this would really simplify my life, and I think it would help others too.
Is there anything blocking it?

[racinmat]

@holtskinner is there anyone else who could review it instead?