Closes the session — no further turns accepted.

Idempotent from the client's view; a new conversation means a fresh init. Rejects a token that doesn't match the route session (403).

curl -X POST "https://api.fruxon.com/v1/embed/sessions/string:close"

{
  "type": "string",
  "title": "string",
  "status": 0,
  "detail": "string",
  "instance": "string",
  "property1": null,
  "property2": null
}