[Feat]: Callback for completed sendMessage request

### Is your feature request related to a problem? Please describe.

When I implement the a2a client and send a message/prompt to the remote agent (LLM), then I want to know when my request has been handled completely. With the hello world example server, I get one message event and that is all. In case of streaming I have no reliable way to determine if I should expect further events or if the connection has already been closed.

### Describe the solution you'd like

I just need a reliable event/callback/notification that is triggered when the processing of my request has finalized (completed, failed, connection interrupted...). It could be another consumer that I can add as a third consumer parameter, it could be a new kind of ClientEvent, the sendMessage method could return a CompletableFuture object, there could be a reactive interface using Mono/Flux from Project Reactor...

### Describe alternatives you've considered

_No response_

### Additional context

_No response_

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