Update check on min quantity for CI Minutes

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Related to #366430 (closed).

This MR updates the check on the add-ons purchase quantity to ensure it is an integer and it's >= 1. The validation is reflected in the updated error message. This applies to Storage purchases as well.

Note that, Storage purchases, an error message appears because when changing the quantity, we make a call, and that call fails. For that, there's a follow-up MR (!105799 (merged)).

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CI Minutes
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