test(core): add automated test suite

[usernane]

Problem

No automated tests in the repository.

Suggestion

Add tests using Jest or Mocha with a mock server.

Test Coverage Should Include

• All HTTP methods (GET, POST, PUT, DELETE)
• Callback execution order
• Error handling scenarios
• CSRF token extraction
• Retry mechanism
• Parameter serialization
• URL validation

Example Structure

/tests
  ajax.test.js
  callbacks.test.js
  csrf.test.js
  retry.test.js

Recommended Setup

{
  "scripts": {
    "test": "jest"
  },
  "devDependencies": {
    "jest": "^29.0.0",
    "jest-environment-jsdom": "^29.0.0"
  }
}