fix(test): use mockReturnValue for synchronous fs mocks

[Copilot]

fs.readdirSync and fs.readFileSync mocks in the FileSystemReader spec were using mockResolvedValue, which wraps the return in a Promise. Since these are synchronous APIs, the unit under test would receive a Promise instead of the expected string/string[], masking real behavior.

Change

// Before
vi.mock('fs', () => ({
  readdirSync: vi.fn().mockResolvedValue([]),
  readFileSync: vi.fn().mockResolvedValue('content'),
}));

// After
vi.mock('fs', () => ({
  readdirSync: vi.fn().mockReturnValue([]),
  readFileSync: vi.fn().mockReturnValue('content'),
}));

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