[Hotfix] Preset expiration in renewals for plans with no periodicity

[lucasdotvin]

This pull request introduces a fix to the renew method in the Subscription model to handle cases where the expirationDate is null, and adds new test cases to ensure the behavior is correctly validated. The changes improve the robustness of subscription renewal logic, particularly for plans with grace days but no periodicity.

Fix to subscription renewal logic:

• src/Models/Subscription.php: Updated the renew method to include a conditional check ensuring that grace days are only calculated if an expirationDate is provided. This prevents potential errors when the expirationDate is null.

New test cases for subscription renewal:

• tests/Models/SubscriptionTest.php: Added two new test cases:
  • testModelRenewsEvenIfPlanHasNoPeriodicityButHasGraceDays: Verifies that a subscription can renew even if the plan lacks periodicity but includes grace days.
  • testModelRenewsWithDefinedExpirationEvenIfPlanHasNoPeriodicityButHasGraceDays: Ensures that a subscription renews correctly with a defined expiration date, calculating the grace_days_ended_at field as expected.