[ai] Automatically just `send` streamed messages if in group-scope

[heyitsaamir]

We currently do some level of accumulation for messages anyway.
For group scope, we have to tell the user to do a check:
if activity.conversation.isGroup, then send
Otherwise, you can stream.

This leads to poor devex. We can abstract this away for them:
In group scopes, we accumulate all the chunks for them. Then once streaming is done, we can just send it for them.