fix(server,sdk): a2a security schemes in agent card

[jezekra1]

Summary

Add platform proxy auth schemes to agent card schemes:

        "security": [
          {
            "platform_context_token": []
          },
          {
            "platform_context_token": [],
            "basic": []
          }
        ],
        "securitySchemes": {
          "platform_context_token": {
            "bearerFormat": "JWT",
            "description": "Platform context token, issued by the AgentStack server using POST /api/v1/context/{context_id}/token.",
            "scheme": "bearer",
            "type": "http"
          },
          "basic": {
            "bearerFormat": null,
            "description": null,
            "scheme": "basic",
            "type": "http"
          }
        },

Linked Issues

Documentation

• No Docs Needed:

If this PR adds new feature or changes existing. Make sure documentation is adjusted accordingly. If the docs is not needed, please explain why.

[gemini-code-assist]

Summary of Changes

Hello @jezekra1, 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 refactors and standardizes the declaration of authentication security schemes within the AgentStack server and SDK, particularly for agent cards. It introduces a new SdkAuthenticationBackend interface, enabling authentication providers to dynamically inject their supported security mechanisms, such as platform_context_token (JWT bearer), into the agent's metadata. This change ensures that agent cards accurately communicate their authentication requirements, improving interoperability and security configuration.

Highlights

• Standardized Agent Card Security: Introduces a consistent way to declare authentication schemes for agent cards, specifically adding platform_context_token as a bearer JWT scheme.
• Authentication Backend Interface: Defines a new abstract base class SdkAuthenticationBackend to allow custom authentication backends to programmatically update agent card security metadata.
• Refactored Authentication Handling: Modifies the SDK's create_app function to remove direct AuthenticationMiddleware setup, implying that authentication configuration is now handled more dynamically via the SdkAuthenticationBackend.
• Proxy and Deployment Agent Card Updates: Ensures that both proxy and deployment agent cards correctly reflect the new platform_context_token and other configured authentication schemes (OIDC bearer, basic).

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[gemini-code-assist]

Code Review

This pull request refactors authentication handling in the SDK to allow authentication backends to modify the agent card's security schemes, which is a solid architectural improvement. It correctly introduces an abstract base class for this purpose. However, there is a critical flaw in how the proxy server combines its own security requirements with those of the upstream agent. The current implementation incorrectly requires clients to authenticate with both the proxy and the agent simultaneously, which defeats the purpose of a proxy. I've provided a detailed comment and a code suggestion to rectify this by ensuring the proxy advertises its own authentication schemes, replacing the agent's.