feat(provider): add vercel to all gateway-supported fallback entries

Adds vercel to every fallback entry where the model exists on the
gateway (kimi-k2.5, glm-5, glm-4.6v, minimax-m2.7, grok-code-fast-1,
gpt-5-nano). Adds inferSubProvider mappings for minimax, moonshotai,
and zai. Updates librarian/explore special cases to prefer minimax
via gateway over claude-haiku.