Runs the same projection the worker publishes through, against the same connections, and writes nothing: no post, no deliveries, no queue job, no metered usage, no media fetch.
Returns per target the text as that platform would receive it, which media it will not take and why, and every warning. This is what a composer draws.
Media given as a url is not downloaded — its type is assumed from the URL and its size is not checked. Those refs come back in unverified_media so the preview never claims to have looked at something it has not.