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

src/server/api/contracts/todo.contract.test.ts

@@ -0,0 +1,20 @@
+import { describe, expect, test } from 'bun:test'
+import { createInsertSchema } from 'drizzle-zod'
+import { generatedFieldKeys } from '@/server/db/fields'
+import { todoTable } from '@/server/db/schema'
+
+describe('todo insert schema', () => {
+  const insertSchema = createInsertSchema(todoTable).omit(generatedFieldKeys)
+
+  test('accepts a minimal valid input', () => {
+    expect(insertSchema.safeParse({ title: 'buy milk' }).success).toBe(true)
+  })
+
+  test('rejects missing title', () => {
+    expect(insertSchema.safeParse({}).success).toBe(false)
+  })
+
+  test('rejects non-string title', () => {
+    expect(insertSchema.safeParse({ title: 42 }).success).toBe(false)
+  })
+})