Retrieves a traffic source if all campaigns use the same one.

Takes a list of campaign ids and returns their traffic source, but only when every campaign in the list uses the same one. Responds 200 with an empty body when they do not — and also when the request body omits the 'ids' key, so an empty response can mean a malformed body rather than a genuine mismatch. Send exactly {"ids": ["<campaign uuid>", ...]}; other key names are ignored rather than rejected.