🧪 [testing] Test askCrystalBall response generation

[marcin2121]

🎯 What: The testing gap addressed
The askCrystalBall function in lib/magic.ts generates responses by randomly selecting from an array of predefined jokes. Before this PR, there were no automated tests ensuring this function returned a valid response from the expected set, nor verifying its asynchronous behavior correctly.

📊 Coverage: What scenarios are now tested
This PR introduces a test in __tests__/lib/magic.test.ts using the built-in Node.js node:test test runner. The test covers the primary "happy path", ensuring that the askCrystalBall function resolves its promise and returns a string that is strictly present within the hardcoded array of jokes.

✨ Result: The improvement in test coverage
Test coverage is improved by validating the fundamental logic and async nature of askCrystalBall. It guarantees that future refactoring (like changing the list of jokes or modifying the selection logic) will be caught if it accidentally breaks the response contract.

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PR created automatically by Jules for task 7159304649656863289 started by @marcin2121

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