Widget SSE events

When stream: true on widget messages, events include:

Event	Purpose	Example payload
step_message	Progress text	{ "message": "Thinking…" }
tool_call	Tool started	{ "name": "get_offers", "message": "Running get_offers…" }
tool_result	Tool finished	{ "name": "get_offers", "success": true }
done	Final response	Message with content, optional rich_content
error	Failure	{ "message": "…" }