chore: Migrate to vitest 4

[Copilot]

Migrate from vitest 3.2.4 to 4.0.6.

Mock Setup Changes

Vitest 4 requires factory functions for vi.mock() to properly initialize mocked modules:

// Before
vi.mock("fs/promises");
const mockFs = vi.mocked(fs);

// After
vi.mock("fs/promises", () => ({
  default: { readFile: vi.fn(), writeFile: vi.fn() },
  readFile: vi.fn(),
  writeFile: vi.fn(),
}));
const mockFs = fs;

Updated add-new-setting.spec.js and update_translation.spec.js accordingly.

Strict Function Wrapping

Vitest 4 enforces that vi.fn() wraps actual functions or classes. Replaced arrow functions with function expressions:

// Before
window.FileReader = vi.fn(() => mockFileReader);
global.Blob = vi.fn((content, options) => mockBlob);

// After
window.FileReader = function() { return mockFileReader; };
global.Blob = vi.fn(function(content, options) { return mockBlob; });

Updated UploadFileComponent.spec.js and export.spec.js.

Setup File Handling

Renamed vite.config.setup.test.js → vite.config.setup.js to prevent vitest from treating it as a test file (new behavior in v4).

Original prompt

Can you migrate vitest to vitest 4 ?

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