test: 启用 bun test 并补 todo contract 示例

- package.json 加 "test": "bun test"
- todo.contract.test.ts 给 starter 一个可复制的 colocated 测试样板
  覆盖 valid input / missing field / wrong type 三种 case

package.json

@@ -21,6 +21,7 @@
     "db:studio": "drizzle-kit studio",
     "dev": "bunx --bun vite dev",
     "fix": "biome check --write",
+    "test": "bun test",
     "typecheck": "tsc --noEmit"
   },
   "dependencies": {