Feat]: Client-side utility for assembling artifacts from update events

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

1.0 release removed `ClientTaskManager` (see #734) and changed client API in a way that it now closely mirrors server API and uses [`StreamResposne`](https://a2a-protocol.org/latest/specification/#323-stream-response) as a return type.

#734 mentions a possible helper for reconstructing artifacts specifically, this issue is to track it.

### Describe the solution you'd like

As stated in #734 maintaining the whole `Task` state locally can result in divergence with server state and should not be relied on by default (especially due to history behavior specification in 1.0, #734 has more details).

For artifacts though the logic is deterministic and can be replicated locally if one wants to maintain full artifact state by interpreting `append` and `lastChunk` flags. Client API should not be changed here, new helper should operate on a stream of `StreamResponse`.