Unit Testing Zustand Stores in E-commerce Apps using Vitest

Act as a Lead QA Automation Engineer. Write a comprehensive, bulletproof test suite utilizing `Vitest` specifically targeting complex business logic within a Zustand e-commerce store. Target the highly critical [ACTION_NAME] (e.g., `applyDiscountCode`). Explicitly define the exact [EXPECTED_STATE_CHANGE] assertions (e.g., total drops by 20%, error state is null). Write the exact testing mocks required to intercept and stub the [MOCK_SERVICES] (like external Stripe APIs or `window.localStorage`). Crucially, demonstrate the exact `beforeEach` setup code required to forcefully reset the Zustand store to its initial state between every single test execution, mathematically preventing test state leakage and ensuring absolute test isolation. Embed a specific test simulating a 500 Network Failure to guarantee the store's error handling resolves correctly.