Vercel AI SDK Streaming Chat (Production)

Act as a senior full-stack engineer. Implement a production-ready streaming chat using Vercel AI SDK v4+ for [FRAMEWORK: Next.js App Router | Astro API route]. Requirements: - Route handler with `streamText`, tool calling optional ([TOOLS: yes/no]) - Client UI with `useChat`, optimistic messages, abort controller, retry on network fail - Message parts: text, tool invocations, error states; persist to [DB] with user/session IDs - Rate limit: [N] req/min per IP; max tokens per request - Env vars list (no secrets in repo) - Testing notes: mock model in CI, snapshot tool-call payloads Deliver: file tree, complete code for server + client, and 5-line security checklist (CSRF, SSRF on tools, prompt injection). Product: [APP_NAME]. Model: [MODEL].