fix: update auth-simple test to mock headers function

src/__tests__/auth-simple.test.ts

@@ -6,6 +6,9 @@ vi.mock('next/headers', () => ({
   cookies: vi.fn().mockResolvedValue({
     get: vi.fn().mockReturnValue({ name: 'better-auth.session_token', value: 'test-token' }),
   }),
+  headers: vi.fn().mockResolvedValue({
+    get: vi.fn().mockReturnValue(null),
+  }),
 }))
 
 // Mock fetch