🧹 Improve error message clarity in Solver#fizzbuzz

[cancelei]

🎯 What: The error message in lib/solver.rb:22 was improved.
💡 Why: It now specifies which argument is invalid ('numb must be an integer' instead of just 'must be an integer'), which improves maintainability and consistency with other methods in the class.
✅ Verification: Verified by running a manual test script that catches the exception and prints the message.
✨ Result: Improved clarity of error messages in the fizzbuzz method.

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PR created automatically by Jules for task 6837448794378505155 started by @cancelei

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