Harden paid registration flow and add unit tests

src/PowderCoating.Application/Interfaces/IStripeService.cs

@@ -4,6 +4,7 @@ public interface IStripeService
 {
     Task<string> CreateCheckoutSessionAsync(int companyId, int newPlan, bool isAnnual, string successUrl, string cancelUrl);
     Task<string> CreateRegistrationCheckoutSessionAsync(int plan, bool isAnnual, string email, string companyName, string successUrl, string cancelUrl);
+    Task<bool> IsRegistrationCheckoutPaidAsync(string sessionId);
     Task FulfillCheckoutAsync(string sessionId);
     Task FulfillRegistrationCheckoutAsync(string sessionId, int companyId, int plan);
     Task SyncSubscriptionAsync(int companyId);