Check billing interval and period to set in mandate options

[Mayisha]

Fixes #3634

In the issue, it is reported that subscription purchases and renewals failed due to empty values in the interval and interval_count args in the mandate_options. We could not reproduce the bug. One possibility is this might be related to a bigger issue with the HPOS data synchronizer causing object metadata to be wiped/deleted (see woocommerce/woocommerce#53307).

Changes proposed in this Pull Request:

• Checking if the subscription has billing interval and billing period data. If these are not present we fallback to the else block to use interval: sporadic.

Testing instructions

As the originally reported bug is not reproducible, test subscription and renewal flow to ensure there are no regressions.
Single subscription

• As a shopper, add a subscription product to your cart and complete the checkout with a card.
  • Check the logs and confirm that interval and interval_count are correctly set in the mandate_options.
• Renew this subscription as an admin from the subscription edit page.
  • Check the logs and confirm that interval and interval_count are correctly set in the mandate_options.
• Renew this subscription as the shopper from 'My Account > Subscriptions' page.
  • Check the logs and confirm that interval and interval_count are correctly set in the mandate_options.

Multiple Subscriptions

• As a shopper, add two subscription products to your cart having different billing intervals and complete the checkout with a card.
  • Check the logs and confirm that interval: sporadic is set in the mandate_options with no interval_count.
• Renew one of the subscriptions as an admin from the corresponding subscription edit page.
  • Check the logs and confirm that interval and interval_count are correctly set in the mandate_options.
• Renew one of the subscriptions as the shopper from 'My Account > Subscriptions' page.
  • Check the logs and confirm that interval and interval_count are correctly set in the mandate_options.

[annemirasol]

Adding this check makes sense to me! 🚢

✅ Correct interval and interval count for single subscription orders, customer-initiated and admin-initiated renewals.
✅ Correct interval and interval count for multiple subscription orders, customer-initiated and admin-initiated renewals.

📝 Reminder to add changelog and readme entries ✏️