Add unit tests for lib modules with Contentful mocking

[Copilot]

Added unit tests for 8 modules lacking coverage in the lib folder. All Contentful SDK calls are mocked to enable isolated testing.

Modules Tested

• backend.js - Migration storage (tag/content), version tracking
• config.js - Configuration loading, confirmation prompts
• contentful.js - API utilities (ID extraction, content naming)
• diff.js - Content comparison across all field types
• environment.js - Environment CRUD operations
• helpers/locale.js - Locale detection and defaults
• tree.js - Content tree building with circular reference handling
• version.js - Version add/delete operations

Test Coverage

• 129 tests across 10 suites
• Error paths extensively covered per requirements
• Console output mocked using jest.spyOn for proper test isolation

Modules Not Tested

Three modules use dynamic imports incompatible with Jest without --experimental-vm-modules:

• doc.js - imports markdown-table dynamically
• migration.js - imports contentful-migration and executes external CLI
• content.js - complex integration with inquirer and contentful-import

These would require either refactoring for testability or integration test approach.

Example Test Pattern

// Mock Contentful SDK
jest.mock('./contentful');

describe('storeMigration', () => {
  beforeEach(() => {
    mockClient.getTag.mockResolvedValue({ name: '100', update: jest.fn() });
  });

  it('should create new tag when tag does not exist', async () => {
    mockClient.getTag.mockRejectedValue(new Error('Not found'));
    await storeMigration({ version: 123, state: STATE_SUCCESS }, mockConfig);
    expect(mockClient.createTag).toHaveBeenCalledWith('migration', '123');
  });
});

Original prompt

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This section details on the original issue you should resolve

<issue_title>Add unit tests</issue_title>
<issue_description>Currently the project has a unit test setup, but only very few functions are actually tested.

We need to add unit tests for most of the modules in the lib folder.
The unit tests should mock the calls to all imported packages that are Contentful related.
Loggin does not need to be tested.
But especially the error cases need to be covered by the unit tests.

</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Add unit tests #81

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