test(auth): add test-only auth instance with testUtils plugin and initial tests

lib/auth/__tests__/auth.test.ts

@@ -0,0 +1,27 @@
+import { describe, it, expect, beforeAll } from "vitest"
+import { testAuth, getTestHelpers } from "@/lib/auth/test"
+import type { TestHelpers } from "better-auth/plugins"
+
+describe("Better-Auth integration", () => {
+  let test: TestHelpers
+
+  beforeAll(async () => {
+    test = await getTestHelpers()
+  })
+
+  it("should create a user", async () => {
+    const user = test.createUser({ email: "test@example.com" })
+    expect(user.email).toBe("test@example.com")
+  })
+
+  it("should create a session for a user", async () => {
+    const user = test.createUser({ email: "session-test@example.com" })
+    await test.saveUser(user)
+
+    const { session, headers } = await test.login({ userId: user.id })
+    expect(session.userId).toBe(user.id)
+    expect(headers.get("cookie")).toBeTruthy()
+
+    await test.deleteUser(user.id)
+  })
+})