Subscribe to a chat's agent SSE stream (Redis-backed).

Emits the exact same events as POST /agent/response — both sides read from the per-chat Redis Stream written by the producer running the agent.

Query params:

• last_event_id: resume from this Redis stream entry id. Pass the id: of the last SSE event you received to avoid duplicates. When omitted, the subscription starts from "now" — only events produced after the GET request arrives are delivered, no history is replayed.
• after: accepted for backward compatibility and currently ignored. Use last_event_id instead.

Like POST /agent/response, the stream is terminated by a tellers.stream.end event carrying the reason the turn ended. It is also emitted when the subscription finds the chat already finished, so a client that reconnects late still gets a definitive answer instead of an idle socket.