MCP connector: support custom covers for Facebook & Instagram Reels#3166

The REST API supports custom video thumbnails (media[].thumbnails + default_thumbnail), but the MCP connector doesn’t expose them. submit_publer_posts and update_publer_post only accept media_id / media_ids / media_url / media_urls, and network_options can’t change media. In testing, a thumbnail passed through the connector was saved without error but silently dropped. Support has confirmed the gap.

Why it matters: we use an AI assistant (via MCP) to queue weekly interview reels. It can design each cover in Canva and upload it to our Publer library, but every cover still has to be attached by hand in the UI.

Request:

1. A per-video cover parameter on submit_publer_posts and update_publer_post (media library id or public image URL), for networks that support custom covers (Facebook Reels/videos, Instagram Reels, LinkedIn, Pinterest, YouTube).
2. The ability to set or replace the cover on existing drafts and scheduled posts.
3. The current cover returned when reading a post, so it can be confirmed.
4. A clear error when a cover isn’t supported, instead of dropping it silently.