curl --request POST \
  --url https://core.nextmatter.com/api/instances/{id}/complete_step/ \
  --header 'Authorization: <api-key>' \
  --header 'Content-Type: application/json' \
  --data '{
  "step_id": 123,
  "actions": [
    {
      "action_id": 123,
      "input_object": "<any>"
    }
  ],
  "mode": "save"
}'

{
  "name": "",
  "process": "<string>",
  "deadline": "2023-11-07T05:31:56Z",
  "priority": "V",
  "tags": [],
  "step_assignments": [
    {
      "step_id": 123,
      "user_id": 123
    }
  ],
  "url": "<string>",
  "id": 123,
  "started_time": "2023-11-07T05:31:56Z",
  "completed_time": "2023-11-07T05:31:56Z",
  "aborted_time": "2023-11-07T05:31:56Z",
  "last_updated_time": "2023-11-07T05:31:56Z",
  "process_snapshot": {},
  "active_step_ids": [
    123
  ],
  "step_deadlines": [
    {}
  ],
  "public_url": "<string>",
  "stage": "<string>"
}