fix: schedule validation for WP import

[HardeepAsrani]

Summary

Fixes WordPress import schedule saving so 10‑minute intervals persist instead of reverting to one‑time by accepting fractional hours in validation.

Will affect visual aspect of the product

YES/NO

Screenshots

Test instructions

• Make sure the 10 min schedule saves properly.

Check before Pull Request is ready:

• I have written a test and included it in this PR
• I have run all tests and they pass
• The code passes when running the PHP CodeSniffer
• Code meets WordPress Coding Standards for PHP, HTML, CSS and JS
• Security and Sanitization requirements have been followed
• I have assigned a reviewer or two to review this PR (if you're not sure who to assign, we can do this step for you)

Closes https://github.com/Codeinwp/visualizer-pro/issues/575.

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