Returns the catalog of voices available for a voice agent's engine.voice_id, grouped by engine version. A voice is valid only for the engine it is listed under — the prisma-1 and horizon-1 catalogs never overlap, so a voice from one is rejected on the other. In addition, prisma-1 still accepts the four ids it shipped with — leimag, nori, norvet and krats, renamed to helena, rafael, bruna and diego respectively. Those four are deprecated: accepted on create and update so an existing integration keeps working, absent from the GET /voice-agent/voices catalog, and never returned by a read. Existing agents were not affected: the voice they play did not change.