Validate A2A Rust <-> Java interoperability in CSIT

[muscariello]

Summary

Validate interoperability between a2aproject/a2a-rs and a2aproject/a2a-java inside CSIT.

Scope

• identify the shared transports currently supported by both SDKs and define the first validation slice
• add CSIT coverage for Rust client -> Java server flows
• add CSIT coverage for Java client -> Rust server flows
• validate agent card compatibility and protocol negotiation behavior
• validate core unary operations: send_message, get_task, list_tasks, and cancel_task
• validate streaming operations: send_streaming_message and subscribe_to_task
• validate representative metadata propagation and error mapping behavior
• capture incompatibilities and open follow-up issues in the appropriate SDK repo or spec repo when needed

################################as ##################re################################as ########### least one shared transport is validated in both client/server directions where both SDKs support that direction

• core unary and streaming flows are exercised and documented
• any failures are either fixed, explicitly tracked, or documented as unsupported scope

Parent epic: #161