fix: address review feedback for Vercel AI Gateway PR

- Add --vercel-ai-gateway CLI option to install command
- Fix librarian/explore priority: native providers (ZAI, Copilot) now take precedence over Vercel gateway
- Add hasVercelAiGateway to installer no-provider warning conditions
- Add hasVercelAiGateway: false to all test file InstallConfig fixtures
- Fix test expectations for model ID format (claude-opus-4.6 vs claude-opus-4-6)