sendMessage
Configures the behavior of the mocking server to handle message/send requests.
This method simulates the behavior of the A2A protocol's message/send endpoint. It allows defining how the server should respond to a "send message" JSON-RPC request by chaining a response configuration using the responds method of the returned SendTaskBuildingStep.
Return
An instance of SendTaskBuildingStep to define the response behavior for message/send requests.