Provide an API/hook to close streams after INPUT_REQUIRED / AUTH_REQUIRED

[bianjp]

Summary

Follow-up to #975 / #989.

After #756, interrupted states keep the stream open by design, and #989 correctly deprecated the misleading isFinal overloads on requiresInput / requiresAuth. We still need a supported way for server embeddings to complete the stream after emitting INPUT_REQUIRED / AUTH_REQUIRED.

Why

With SSE, only the server can complete the stream. The client can’t cleanly close it (dropping the connection still leaves server-side resources open until timeout). SSE is also one-way, so after INPUT_REQUIRED the client must send follow-up input via a new request anyway — keeping the original stream open doesn’t help that path, and unclosed connections become costly under load.

Prior discussion

• fix(server): close event stream when requiresInput/requiresAuth force isFinal #980 proposed wiring isFinal=true to close the stream; maintainers preferred not to, since isFinal isn’t in the spec and blurs task state vs connection lifecycle (comment).
• Suggested direction in [Bug]: AgentEmitter#requiresInput(..., isFinal=true) does not close the event stream #975: a hook on interrupted-state events plus a dedicated close API, so users can choose whether to keep the stream open or complete it.

Ask

Please consider adding an explicit close/hook API (exact shape TBD). Default can remain “stream stays open” per #756.