InvocationCloseAck

Typed acknowledgement the MCP/CLI surfaces render after a close (the HTTP
endpoint itself returns 204 No Content). Echoes the closed invocation + its
terminal disposition — a structured row rather than inline JSON, shared so
both surfaces emit the same shape.

{
    "disposition": "completed",
    "invocation_id": "9efe53a2-0f4e-415e-a218-b405e1367798"
}