Streaming partial messages for UI integration.

[jackgerrits]

We need to ensure the streaming use case is adequately supported in the API. While agent-agent communication likely does not need streaming, we need to ensure agent-UI supports streaming.

User can:

• Create a UI which can show partial model output
• Interrupt an in progress streaming response

[ekzhu]

We should use a closure agent to collect the partial messages and pump them through to the front end via a websocket.

The partial message type is application defined.

We can show case this using a cookbook or a sample directory.

[ekzhu]

Related to #3970 and #3969, a simple solution is to just have the agent and team return a streaming iterator.