A first integration test of the model call: Workers AI through the AI
binding, behind the same `complete(messages)` interface as the sim's
fake model. Opt-in (`pnpm test:model`) since it needs `wrangler login`
and costs money.
Red: src/model.ts doesn't exist yet.
Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
