[Feature] Improvements for subscriptions with no expiration

[lucasdotvin]

This pull request refines subscription handling by improving the logic for expiration and overdue status, and adds corresponding test cases to ensure correctness. The key changes include adjustments to how expiration dates are determined, handling of overdue status, and new tests to validate these behaviors.

Subscription expiration logic improvements:

• Updated subscribeTo method in HasSubscriptions.php to set expiration to null explicitly if the plan lacks periodicity, and adjusted grace days calculation to account for the absence of expiration.

Overdue status handling:

• Modified getIsOverdueAttribute in Subscription.php to return false if the subscription has no expiration date, ensuring overdue logic is only applied when relevant.

New test cases:

• Added testItUsesReceivedExpirationEvenIfThePlanHasNoPeriodicity in HasSubscriptionsTest.php to verify that an explicitly provided expiration date is respected even when the plan lacks periodicity.
• Added testModelDoesNotRegisterOverdueIfThereIsNoExpiration in SubscriptionTest.php to confirm that overdue status is not marked if the subscription has no expiration date.