Array / multi-account filter on GET /v1/ads/tree (adAccountId[])

Today /v1/ads/tree takes a single adAccountId, which works well server-side. But agency profiles often have an entire Business Manager connected to one Zernio profile — many ad accounts and a large number of campaigns — while a given workspace only owns a few of those accounts.

To scope the tree to several accounts we currently make one request per account and concatenate (Zernio support confirmed this is the intended approach today). That's fine at a couple of accounts, but it's N requests per view and grows linearly as a workspace is scoped to more accounts.

Ask: support a multi-value account filter, e.g. adAccountId=act_A&adAccountId=act_B or adAccountIds[]=..., so scoping to N accounts is a single request. This mirrors Meta's Business Manager model, where working across multiple ad accounts at once is standard.

Related: passing a plural like adAccountIds / platformAdAccountIds today returns 200 with the full unfiltered set instead of a 4xx, so it's easy to assume it filtered when it didn't. A validation error on unknown params would make this safer to catch.