test(good-fences): the model counts the rs in strawberry (red)

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>

good-fences/package.json

@@ -3,9 +3,16 @@
   "private": true,
   "type": "module",
   "scripts": {
-    "test": "node --test test/*.test.ts"
+    "test": "node --test test/*.test.ts",
+    "test:model": "vitest run -c vitest.model.config.ts",
+    "types": "wrangler types -c wrangler.model-tests.jsonc"
   },
   "dependencies": {
     "ses": "^1.14.0"
+  },
+  "devDependencies": {
+    "@cloudflare/vitest-plugin": "1.3.6",
+    "vitest": "4.1.11",
+    "wrangler": "^4.148.0"
   }
 }