Add unit tests for authentication middleware and error handling

[Copilot]

Added comprehensive unit test coverage for authentication/authorization middleware and core error handling utilities.

Test Coverage Added

Authentication Middleware (tests/unit/middlewares/auth.middleware.test.js)

• JWT validation (Bearer token and cookie-based)
• Token expiration and invalid token handling
• Service-to-service authentication via X-Service-Token header
• Role-based access control (requireAdmin, requireRoles)
• User deactivation enforcement

// Example: Service token authentication test
it('should authenticate valid service token', async () => {
  mockReq.headers['x-service-token'] = 'auth-service-token';
  await requireServiceToken(mockReq, mockRes, mockNext);
  
  expect(mockReq.callingService).toBe('auth-service');
  expect(mockNext).toHaveBeenCalledWith();
});

Error Response Class (tests/unit/core/errors.test.js)

• ErrorResponse construction with status codes and error codes
• HTTP error scenarios (400, 401, 403, 404, 409, 500)
• Error throwability and message preservation

Results

• 39 new tests across 2 files
• 249 total tests passing
• 100% coverage on errors.js
• 70.78% coverage on auth.middleware.js

Implementation Notes

• ES6 modules with Jest
• Winston logger mocked to prevent console warnings
• All external dependencies (JWT, MongoDB) properly mocked
• Tests isolated with no external dependencies

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