Add a paused ad set to a Meta campaign

Creates one new ad set, PAUSED, inside a campaign that already exists: its own audience, optimisation goal, budget and schedule. It does not create any ads — add one afterwards with meta_add_ad. Use when: the user wants to test another audience, another geography or another optimisation goal inside the same campaign. Do not use when the campaign holds the budget (campaign budget optimisation): the new ad set must not carry its own budget, and Meta rejects it. The preview says so before anything is sent. Do not pass free text for locations or interests; use meta_search_targeting first. Returns the new ad set id and reads it back so the paused status and budget are confirmed. The preview refuses what Meta would reject: unknown location keys, EU/EEA targeting without dsa_beneficiary and dsa_payor, a budget below Meta's minimum, past dates, and targeting the campaign's special ad category forbids. If the user has not said what the new ad set should target differently, ask — a duplicate of an existing ad set just splits the same budget.