Add a paused ad to a Meta ad set

Creates one ad, PAUSED, inside an ad set that already exists. The creative is either one that already exists (creative_id) or a new single-image creative built from image_url plus copy. Use when: the user wants another ad in an ad set — a new creative to test, or the same creative reused in a second audience. Do not use to create a campaign or an ad set; meta_create_image_campaign and meta_add_ad_set do that. Do not pass both creative_id and image_url: reusing a creative and making one are different intentions and the tool will not guess. Returns the new ad id (and the creative id when one was created) and reads the ad back so the paused status is confirmed. If the user has not said which ad set, ask. Adding an ad to the wrong ad set puts it in front of the wrong audience.