ClientBuilder has no fallback to AgentCard#additionalInterfaces()/preferredTransport() (legacy 0.3 cards) when supportedInterfaces() is absent

[carlesarnal]

Describe the bug

ClientBuilder#getServerInterfacesMap() builds its list of candidate transports from AgentCard#supportedInterfaces() only:

private Map<String, AgentInterface> getServerInterfacesMap() throws A2AClientException {
    List<AgentInterface> interfaces = agentCard.supportedInterfaces();
    if (interfaces == null || interfaces.isEmpty()) {
        throw new A2AClientException("No server interface available in the AgentCard");
    }
    ...
}

If an AgentCard was produced by an A2A "0.3" server (e.g. anything built on io.a2a.spec.AgentCard from io.github.a2asdk:a2a-java-sdk-reference-jsonrpc, which only has additionalInterfaces/preferredTransport, not supportedInterfaces), Client.builder(agentCard) / ClientBuilder#build() always throws A2AClientException: No server interface available in the AgentCard, even though AgentCard itself models exactly this legacy shape via additionalInterfaces():

private final List<AgentInterface> supportedInterfaces;
private final List<Legacy_0_3_AgentInterface> additionalInterfaces;
...
public List<AgentInterface> supportedInterfaces();
public List<Legacy_0_3_AgentInterface> additionalInterfaces();

Since Legacy_0_3_AgentInterface exists specifically to deserialize the older additionalInterfaces/preferredTransport shape, I'd expect ClientBuilder to fall back to it (converting each Legacy_0_3_AgentInterface + the card's preferredTransport into an AgentInterface) when supportedInterfaces is absent, so that clients built against the current SDK can still talk to agents that only publish a 0.3-style card.

How to reproduce

1. Stand up any A2A server that only serves a 0.3-style card, e.g.:

   {
     "name": "Summarizer Agent",
     "url": "http://localhost:10010",
     "version": "1.0.0",
     "capabilities": {"streaming": false, "pushNotifications": false, "stateTransitionHistory": false},
     "defaultInputModes": ["text"],
     "defaultOutputModes": ["text"],
     "skills": [...],
     "additionalInterfaces": [{"transport": "JSONRPC", "url": "http://localhost:10010"}],
     "preferredTransport": "JSONRPC",
     "protocolVersion": "0.3.0"
   }

   (this is exactly what io.a2a.spec.AgentCard.Builder from a2a-java-sdk-reference-jsonrpc 0.3.0.Beta1/0.3.2.Final produces — there's no supportedInterfaces() builder method on that class at all, so no 0.3-based server can add it)
2. AgentCard card = A2A.getAgentCard("http://localhost:10010");
3. Client.builder(card).withTransport(JSONRPCTransport.class, new JSONRPCTransportConfigBuilder()).build();
4. Observe A2AClientException: No server interface available in the AgentCard, even though the server is reachable and does support JSON-RPC.

I confirmed via a minimal Jackson round-trip that a supportedInterfaces entry must use the field name protocolBinding (not transport) to deserialize into AgentInterface, and that capabilities.stateTransitionHistory (present on all 0.3-style cards) is silently ignored rather than rejected — so the parsing itself is otherwise lenient, it's specifically this transport-selection step that has no legacy fallback.

Expected behavior

ClientBuilder should be able to build a working client against an agent that only publishes a 0.3-style card (additionalInterfaces + preferredTransport), by falling back to AgentCard#additionalInterfaces() (converting Legacy_0_3_AgentInterface → AgentInterface, using preferredTransport() to pick the client's preferred one) when supportedInterfaces() is null/empty.

Context

Found this while wiring up langchain4j-agentic-a2a's AgenticServices.a2aBuilder(url) (used by quarkus-langchain4j-a2a-apicurio-registry's ApicurioAgentsRegistry) against real A2A agents built with a2a-java-sdk-reference-jsonrpc 0.3.0.Beta1/0.3.2.Final for a demo project (https://github.com/carlesarnal/agent-discovery-demo). As things stand, no agent built on the "0.3" reference server can be reached by any client built on the current org.a2aproject.sdk Client/ClientBuilder, regardless of how the client obtains the AgentCard.