[PM-19180] Calculate sales tax correctly for sponsored plans (#5611)

* [PM-19180] Calculate sales tax correctly for sponsored plans

* Cannot divide by zero if total amount excluding tax is zero.

* Unit tests for families & families for enterprise

---------

Co-authored-by: Conner Turnbull <133619638+cturnbull-bitwarden@users.noreply.github.com>

src/Core/Billing/Models/Api/Requests/Organizations/PreviewOrganizationInvoiceRequestModel.cs

@@ -1,5 +1,6 @@
 using System.ComponentModel.DataAnnotations;
 using Bit.Core.Billing.Enums;
+using Bit.Core.Enums;
 
 namespace Bit.Core.Billing.Models.Api.Requests.Organizations;
 
@@ -20,6 +21,8 @@ public class OrganizationPasswordManagerRequestModel
 {
     public PlanType Plan { get; set; }
 
+    public PlanSponsorshipType? SponsoredPlan { get; set; }
+
     [Range(0, int.MaxValue)]
     public int Seats { get; set; }