[AC-1910] Allocate seats to a provider organization (#3936)

* Add endpoint to update a provider organization's seats for consolidated billing.

* Fixed failing tests

src/Core/Models/Business/ProviderSubscriptionUpdate.cs

@@ -0,0 +1,61 @@
+using Bit.Core.Billing.Extensions;
+using Bit.Core.Enums;
+using Stripe;
+
+using static Bit.Core.Billing.Utilities;
+
+namespace Bit.Core.Models.Business;
+
+public class ProviderSubscriptionUpdate : SubscriptionUpdate
+{
+    private readonly string _planId;
+    private readonly int _previouslyPurchasedSeats;
+    private readonly int _newlyPurchasedSeats;
+
+    protected override List<string> PlanIds => [_planId];
+
+    public ProviderSubscriptionUpdate(
+        PlanType planType,
+        int previouslyPurchasedSeats,
+        int newlyPurchasedSeats)
+    {
+        if (!planType.SupportsConsolidatedBilling())
+        {
+            throw ContactSupport($"Cannot create a {nameof(ProviderSubscriptionUpdate)} for {nameof(PlanType)} that doesn't support consolidated billing");
+        }
+
+        _planId = GetPasswordManagerPlanId(Utilities.StaticStore.GetPlan(planType));
+        _previouslyPurchasedSeats = previouslyPurchasedSeats;
+        _newlyPurchasedSeats = newlyPurchasedSeats;
+    }
+
+    public override List<SubscriptionItemOptions> RevertItemsOptions(Subscription subscription)
+    {
+        var subscriptionItem = FindSubscriptionItem(subscription, _planId);
+
+        return
+        [
+            new SubscriptionItemOptions
+            {
+                Id = subscriptionItem.Id,
+                Price = _planId,
+                Quantity = _previouslyPurchasedSeats
+            }
+        ];
+    }
+
+    public override List<SubscriptionItemOptions> UpgradeItemsOptions(Subscription subscription)
+    {
+        var subscriptionItem = FindSubscriptionItem(subscription, _planId);
+
+        return
+        [
+            new SubscriptionItemOptions
+            {
+                Id = subscriptionItem.Id,
+                Price = _planId,
+                Quantity = _newlyPurchasedSeats
+            }
+        ];
+    }
+}