refactor: GH-165 Decoupled the transport layer

[sunyuhan1998]

This PR decouples the transport protocol layer. Considering that, according to the A2A Protocol Specification (https://a2aproject.github.io/A2A/latest/specification/#3-transport-and-format), communication must occur over HTTP(S), I believe we can first decouple the transport layer into a package, rather than rushing to split it into a module (which would face some issues).

The following changes have been made in this PR:

1. Abstract out A2ATransport as the transport layer for the A2A protocol.
2. Refactor all existing code that directly used HttpClient to instead use A2ATransport (defaulting to JdkA2AHttpTransport).
3. Add a new constructor to A2AClient that accepts an A2ATransport parameter.
4. All test classes have been executed and passed successfully.

Fixes: #165

[gemini-code-assist]

Summary of Changes

Hello @sunyuhan1998, 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 significantly refactors the A2A client library by decoupling its transport layer. The primary goal is to abstract away the specifics of HTTP communication behind a new A2ATransport interface, making the client more flexible and easier to extend with different transport mechanisms in the future. This change aligns with the A2A Protocol Specification's requirement for HTTP(S) communication while improving the overall architecture and maintainability of the codebase.

Highlights

• Transport Layer Abstraction: Introduced a new A2ATransport interface (client/src/main/java/io/a2a/transport/A2ATransport.java) to abstract the underlying communication mechanism, allowing for greater flexibility and decoupling from specific HTTP client implementations.
• Refactoring of Core Client Classes: The A2A, A2ACardResolver, and A2AClient classes (client/src/main/java/io/a2a/A2A.java, client/src/main/java/io/a2a/client/A2ACardResolver.java, client/src/main/java/io/a2a/client/A2AClient.java) have been refactored to depend on the new A2ATransport interface instead of directly using A2AHttpClient, promoting a more modular design.
• HTTP Transport Implementation Migration: Existing HTTP client components (A2AHttpClient, JdkA2AHttpClient) have been moved to a new io.a2a.transport.http package and renamed to A2AHttpTransport and JdkA2AHttpTransport respectively (client/src/main/java/io/a2a/transport/http/A2AHttpTransport.java, client/src/main/java/io/a2a/transport/http/JdkA2AHttpTransport.java). A2AHttpTransport now extends A2ATransport, and JdkA2AHttpTransport provides the default Java HTTP client implementation for the new interface.
• Consolidation of Request Logic: Common HTTP request patterns for agent card retrieval, message sending, and streaming (getAgentCard, sendMessage, sendMessageStreaming) have been moved from A2ACardResolver and A2AClient into the JdkA2AHttpTransport implementation, centralizing transport-specific logic.
• Enhanced Client Instantiation: A new constructor has been added to A2AClient (client/src/main/java/io/a2a/client/A2AClient.java, lines 82-91) that accepts an A2ATransport parameter, enabling users to inject custom transport implementations.

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[gemini-code-assist]

Code Review

This PR does a great job of decoupling the transport layer by introducing the A2ATransport interface and moving the HTTP-specific logic into JdkA2AHttpTransport. The code is much cleaner and more modular as a result.

My review focuses on a few key areas for improvement:

• A performance issue in the new streaming implementation that causes an unnecessary serialization/deserialization cycle.
• Improving the clarity of the new A2ATransport API by renaming some parameters.
• Enhancing test code quality by simplifying the test doubles and reducing code duplication.

Overall, this is a solid refactoring. Addressing these points will make the implementation more robust and maintainable.

[sunyuhan1998]

Hi @kabir @maeste , what do you think about this? Could you help review this PR? thank you!

[kabir]

@sunyuhan1998 Hi, just trying to get an alpha release ready and deployed to Maven. This changes the groupId, so I think it is best to wait with this until after that is merged and released

[sunyuhan1998]

@sunyuhan1998 Hi, just trying to get an alpha release ready and deployed to Maven. This changes the groupId, so I think it is best to wait with this until after that is merged and released

No problem. By the way, I noticed that in that PR you used a temporary groupId—will it be modified in a separate branch (for releasing the alpha version)? Or are we going to use this groupId in the main branch going forward until the GA release?

[kabir]

That still needs some discussion, but I think it is probably easier to have it in main, and then we switch back once we have the proper domain registered for the verification of the final group ID

[sunyuhan1998]

That still needs some discussion, but I think it is probably easier to have it in main, and then we switch back once we have the proper domain registered for the verification of the final group ID

I see, thank you for your explanation!

[kabir]

Hi @sunyuhan1998 I have merged the group id change, and done 0.2.3.Alpha1 and 0.2.3.Beta1 releases.

Can you please rebase your branch on the latest?

[sunyuhan1998]

Hi @sunyuhan1998 I have merged the group id change, and done 0.2.3.Alpha1 and 0.2.3.Beta1 releases.

Can you please rebase your branch on the latest?

Hi @kabir , I've merged the latest code from the main branch into the current PR. Is that okay?