hill_cipher.py: Use int() to gracefully deal with float input

[cclauss]

Describe your change:

Use int(num) instead of round(num) to workaround ruff rule ruf057 unnecessary-round.

• Various ruff fixes #12821 (comment)

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[dhruvmanila]

This call seems unnecessary because the type of num is annotated as int as discussed previously. I leave this up to you on whether you want to continue with this approach or update the type annotation to reflect the function contract.

[cclauss]

The num parameter here should be an int. Python will not prevent the caller from breaking that rule.

Using int(num) instead of round(num) ensures the function remains well-behaved in the face of garbage input.