[npm-check-fork] Add comprehensive unit tests for NpmRegistryClient with http mocking

[Copilot]

Addresses code review feedback requesting test coverage for the custom HTTP client implementation introduced in the package-json and throat dependency replacement.

Changes

Test Coverage: Added 17 unit tests mocking http.request/https.request to validate:

• URL encoding for scoped packages (@scope/name → @scope%2Fname)
• Protocol selection (http vs https)
• Request headers (User-Agent, Accept-Encoding)
• Response decompression (gzip, deflate)
• Timeout handling
• Status code handling (404, 5xx, non-2xx)
• Error paths (network errors, stream errors, parse errors)
• Custom configuration (registry URL, user agent, timeout)

Implementation

Tests use Jest mocks and EventEmitter to simulate HTTP request/response lifecycle without network calls:

const mockData: INpmRegistryPackageResponse = {
  name: 'test-package',
  versions: { '1.0.0': { name: 'test-package', version: '1.0.0' } },
  'dist-tags': { latest: '1.0.0' }
};

mockHttpsRequest.mockImplementation((options, callback) => {
  setImmediate(() => callback(response as http.IncomingMessage));
  return request;
});

const fetchPromise = client.fetchPackageMetadataAsync('test-package');

response.statusCode = 200;
setImmediate(() => {
  response.emit('data', Buffer.from(JSON.stringify(mockData)));
  response.emit('end');
});

const result = await fetchPromise;
expect(result.data).toEqual(mockData);

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