Boost existing Instagram Reels/videos via /v1/ads/boost (newer creative path)

Currently POST /v1/ads/boost promotes an existing post via the object_story_id path. For existing Instagram videos/Reels this makes Meta return error 1815279 ("When advertising an existing Instagram video, you need to upload it to Facebook before creating the ad"), so video posts cannot be boosted at all — image and carousel posts work fine.

Meta's Marketing API supports boosting existing IG Reels through the newer creative path (instagram_user_id + source media reference). Request: wire this path into /ads/boost so existing Reels can be promoted directly, keeping the organic post's social proof (likes/comments).

Context: we run a social planner for coaching clients whose content is heavily Reels-based. Right now the boost endpoint covers only a minority of their published posts. We're aware of the /v1/ads/create workaround (building a dark ad from the source video), but that loses the organic post's engagement, which is the main reason to boost in the first place.