List Vendors

List the valid vendor identifiers + display names + host mapping.

Price-enrichment keys off the INTERNAL vendor enum (TWENTYONEAT, CGSTL, VANTOR, SIIS, SPACEWILL, IMPROSAT, ESA) — a business name like "21AT" silently prices at $0. This endpoint makes the right identifier discoverable so callers don't have to guess: each entry carries the vendor enum (what pricing expects), the catalog host (what the search endpoints use on the wire), a human displayName, and the constellation.