[PM-25088] - refactor premium purchase endpoint (#6262)

* [PM-25088] add feature flag for new premium subscription flow

* [PM-25088] refactor premium endpoint

* forgot the punctuation change in the test

* [PM-25088] - pr feedback

* [PM-25088] - pr feedback round two

src/Core/Billing/Services/ILicensingService.cs

@@ -26,4 +26,5 @@ public interface ILicensingService
         SubscriptionInfo subscriptionInfo);
 
     Task<string?> CreateUserTokenAsync(User user, SubscriptionInfo subscriptionInfo);
+    Task WriteUserLicenseAsync(User user, UserLicense license);
 }

src/Core/Billing/Services/Implementations/LicensingService.cs

@@ -389,4 +389,12 @@ public class LicensingService : ILicensingService
         var token = tokenHandler.CreateToken(tokenDescriptor);
         return tokenHandler.WriteToken(token);
     }
+
+    public async Task WriteUserLicenseAsync(User user, UserLicense license)
+    {
+        var dir = $"{_globalSettings.LicenseDirectory}/user";
+        Directory.CreateDirectory(dir);
+        await using var fs = File.OpenWrite(Path.Combine(dir, $"{user.Id}.json"));
+        await JsonSerializer.SerializeAsync(fs, license, JsonHelpers.Indented);
+    }
 }

src/Core/Billing/Services/Implementations/PremiumUserBillingService.cs

@@ -304,7 +304,7 @@ public class PremiumUserBillingService(
         {
             new ()
             {
-                Price = "premium-annually",
+                Price = StripeConstants.Prices.PremiumAnnually,
                 Quantity = 1
             }
         };

src/Core/Billing/Services/NoopImplementations/NoopLicensingService.cs

@@ -73,4 +73,9 @@ public class NoopLicensingService : ILicensingService
     {
         return Task.FromResult<string?>(null);
     }
+
+    public Task WriteUserLicenseAsync(User user, UserLicense license)
+    {
+        return Task.CompletedTask;
+    }
 }