List reusable 10DLC brands

Lists the account's 10DLC brands that a **new campaign** can be registered under, for use as `brandId` when submitting a registration. A brand is the business identity carriers vet; a campaign is one messaging use case beneath it. One brand can carry several campaigns, so a second use case — say event reminders alongside order confirmations — is a new campaign on the brand you already have, not a second registration of the same business. Reusing the brand keeps its approval and its vetting score, and skips the wait for both. Two conditions decide what appears here, and both come from the carrier registry rather than from Dial: - **The brand is approved.** The registry refuses a campaign filed against a brand it hasn't approved yet, so a brand still in review is not offered. - **The brand is a business.** A sole proprietor brand covers exactly one use case and one number, so it can never carry a second campaign. An account with nothing to reuse gets an empty list — that's a successful answer, not a `404`.