Add an ad group to a TikTok campaign (paused)

Creates one more ad group inside an existing TikTok campaign, PAUSED, with its own budget, schedule and targeting. Use when: the user wants to test a second audience, split budget across markets, or add a market to a campaign that already works. The campaign's objective decides what the ad group may optimise for, so nothing has to be chosen twice. Do not use it to change an existing ad group — that is tiktok_update_ad_group. Do not create near-duplicate ad groups targeting overlapping audiences: on TikTok they bid against each other and both get more expensive. Do not add an ad group and expect it to deliver: it is created paused and it has no ads. Add one with tiktok_add_ad, then resume both. Validates before creating: the optimisation goal against the campaign's objective, the ad group budget range for the account's currency, the bid against its limits and both budgets, the schedule (the advertiser's days; a past start is refused), the pixel and event when the objective needs them (the pixel must be on the account and receive that event), every location id and language code against TikTok's lists (overlapping locations and the under-18 rules included), and the campaign's special ad category limits. Returns the new ad group id, read back from TikTok.