Fixed prime check loop to avoid integer overflow

[Varshap2205]

Description of Change

Changed the loop condition in is_prime() from i * i <= num to i <= num / i to prevent potential integer overflow for large values of num.

The logic of the algorithm remains the same, and all primes and non-primes are correctly identified. Tested with edge cases: 2, 3, 5, 9, 25, 121.

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