refactor: move mockLLM to core (#1493)

.changeset/perfect-turtles-mate.md

apps/next/src/app/api/chat/route.ts

apps/next/src/components/demo/chat/rsc/ai-action.tsx

apps/next/src/lib/utils.ts

packages/core/src/utils/index.ts

packages/core/src/utils/llms.ts