feat: Add 0.3 protocol version compatibility layer

[kabir]

This passes the 0.3 TCK, and all unit tests.
At the moment 0.3 and 1.0 are NOT co-hosted, you need to select one

[ehsavoie]

/gemini review

[gemini-code-assist]

Code Review

This pull request introduces a comprehensive backward compatibility layer for A2A Protocol v0.3, enabling interoperability between v1.0 SDK components and v0.3 agents. The implementation includes dedicated spec types, a conversion layer using MapStruct, and transport handlers for JSON-RPC, gRPC, and REST. Key feedback includes the need to handle authentication errors in the JDK HTTP client's GET method, fixing misleading error messages in asynchronous requests, and enhancing the SSE parser's robustness. Furthermore, the REST transport requires consistent stream completion signaling, and lazy initialization of the agent card must be made thread-safe to prevent race conditions.

[gemini-code-assist]

The get() method does not check for HTTP_UNAUTHORIZED (401) or HTTP_FORBIDDEN (403) status codes, unlike the post() method (lines 270-274). This inconsistency means authentication errors during GET requests (like fetching the agent card) might not be reported with the standard A2A error messages, violating the A2A protocol specification for error code mappings.

        @Override
        public A2AHttpResponse_v0_3 get() throws IOException, InterruptedException {
            HttpRequest request = createRequestBuilder(false)
                    .build();
            HttpResponse<String> response =
                    httpClient.send(request, BodyHandlers.ofString(StandardCharsets.UTF_8));

            if (response.statusCode() == HTTP_UNAUTHORIZED) {
                throw new IOException(A2AErrorMessages.AUTHENTICATION_FAILED);
            } else if (response.statusCode() == HTTP_FORBIDDEN) {
                throw new IOException(A2AErrorMessages.AUTHORIZATION_FAILED);
            }

            return new JdkHttpResponse(response);
        }

References

1. Adhere to the A2A protocol specification for error code mappings, even if other mappings seem more internally consistent.

[gemini-code-assist]

In asyncRequest, the bodyHandler is defined as BodyHandler<Void>, which means response.body() will always return null. Including it in the error message is misleading and provides no useful information.

                            subscriber.onError(new IOException("Request failed with status " + response.statusCode()));

[gemini-code-assist]

Code Review

This pull request introduces a comprehensive backward compatibility layer for A2A protocol version 0.3, enabling interoperability between v1.0 SDK components and v0.3 agents. The changes include a dedicated module structure for spec types, a server-side conversion layer using MapStruct, and transport-specific client and server implementations. Feedback highlights several critical issues: a missing checked exception declaration in the REST transport, incorrect resource name patterns and stubbed methods in the gRPC transport, and inconsistent field initialization. Additionally, a potential null pointer risk was identified in the SSE event listener's error handling logic.

[gemini-code-assist]

The getAgentCard method is currently a stub that just returns the cached agentCard. Unlike the JSON-RPC and REST transports, it does not attempt to fetch the card from the server or handle the authenticated extended card logic. It should be implemented using the gRPC GetAgentCard method to ensure consistency across transports.

[gemini-code-assist]

Passing null to the errorHandler to signal completion is risky. If the consumer implementation of errorHandler (which is a Consumer<Throwable>) does not explicitly check for null, it will throw a NullPointerException. It is better to use a separate completion callback or ensure the error handler handles nulls.