feat: Add 0.3 compatibility layer

[kabir]

This passes the reference/* tests

[gemini-code-assist]

Code Review

This pull request introduces a v0.3 compatibility layer for the A2A SDK, including new client transport implementations (gRPC, JSON-RPC, REST) and a server-side conversion layer to map v0.3 requests to v1.0. The review identified several issues: the SSE implementation in JdkA2AHttpClient is fragile regarding message boundaries, the A2A.getAgentCard method inefficiently creates new HTTP clients, the GrpcTransport.close() method fails to shut down the channel, the gRPC getAgentCard implementation lacks support for extended cards, and the JSONRPCTransport has a potential race condition in its SSE event listener setup. Additionally, an improvement was suggested to include response bodies in HTTP error messages.

[gemini-code-assist]

The SSE implementation in onNext is fragile. It assumes that each line starting with data: is a complete and independent message. However, the SSE specification allows a single message to span multiple data: lines, and messages are only considered complete when an empty line (end of event) is encountered. Additionally, this implementation ignores other SSE fields like event:, which might be important for certain transports.

Consider accumulating data: lines in a buffer and only triggering the messageConsumer when an empty line (end of event) is encountered.

[gemini-code-assist]

Creating a new JdkA2AHttpClient instance for every call to getAgentCard is inefficient. HttpClient (which is wrapped by JdkA2AHttpClient) is designed to be long-lived and reused across multiple requests to benefit from connection pooling and shared resources.

Consider using a shared static instance of JdkA2AHttpClient or allowing one to be passed in.

[gemini-code-assist]

The close() method is currently a no-op. If the gRPC channel was created specifically for this transport (e.g., via the factory in GrpcTransportProvider), it should be shut down here to avoid resource leaks.

Check if the channel is an instance of ManagedChannel and call shutdown() if it is.

    @Override
    public void close() {
        if (asyncStub.getChannel() instanceof io.grpc.ManagedChannel managedChannel) {
            managedChannel.shutdown();
        }
    }

[gemini-code-assist]

The getAgentCard implementation for gRPC currently ignores the ClientCallContext and returns the cached agentCard. This prevents the client from retrieving an 'authenticated extended card' even if the server supports it and the context contains the necessary credentials.

Unlike the JSON-RPC and REST transports, this implementation lacks the logic to fetch the extended card when requested.

[gemini-code-assist]

There is a potential race condition here. postAsyncSSE might trigger the messageConsumer (and thus sseEventListener.onMessage) before the ref.set() call completes. If this happens, ref.get() inside the lambda will return null, leading to a NullPointerException in SSEEventListener when it tries to cancel the future.

Consider ensuring the future is available to the listener before any data can be processed.

[gemini-code-assist]

When an HTTP request fails with a non-success status code, the error message only includes the status code. It would be much more helpful for debugging if the response body (which often contains error details in JSON format) was included in the exception message.

Suggested change

	throw new IOException("Request failed " + response.status());
	throw new IOException("Request failed " + response.status() + ": " + response.body());