List upcoming open posting slots

Requires scope: `schedules:read`. Returns upcoming schedule slots not yet filled by a post, across your connected accounts.

curl -X GET "https://example.com/posting_schedules/open_slots"

{  "open_slots": [    {      "scheduled_at": "2026-07-23T13:00:00Z",      "social_account_id": 301,      "platform": "twitter"    }  ]}