ReqManagedAgents.Provider for the Anthropic Managed Agents backend — :streaming mode.
A long-lived SSE stream pushes events; the client POSTs events to drive it; a turn ends on
session.status_idle. Resume POSTs a user.custom_tool_result event (no echo — the session
already holds the tool call). Composes Client, Stream, Event.
normalize/1 keys off the most recent status event. events, text, and server_tool_uses
reflect exactly the events passed in (a partial list yields a partial view).