Communicate with an assistant using the Agent-to-Agent Protocol. Sends a JSON-RPC 2.0 message to the assistant. - Request: Provide an object with jsonrpc, id, method, and optional params. - Response: Returns a JSON-RPC response with task information or error. Supported Methods: - message/send: Send a message to the assistant - tasks/get: Get the status and result of a task Notes: - Supports threaded conversations via thread context - Messages can contain text and data parts - Tasks run asynchronously and return completion status