[PM-29556] Fix: changing organization plan nulls out public and private keys (#6738)

Main fix: only assign new key value where old keys are not set
and new keys have been provided.

Refactors:
- use consistent DTO model for keypairs
- delete duplicate property assignment for new orgs

src/Api/Billing/Models/Requests/KeyPairRequestBody.cs

@@ -2,6 +2,7 @@
 #nullable disable
 
 using System.ComponentModel.DataAnnotations;
+using Bit.Core.KeyManagement.Models.Data;
 
 namespace Bit.Api.Billing.Models.Requests;
 
@@ -12,4 +13,11 @@ public class KeyPairRequestBody
     public string PublicKey { get; set; }
     [Required(ErrorMessage = "'encryptedPrivateKey' must be provided")]
     public string EncryptedPrivateKey { get; set; }
+
+    public PublicKeyEncryptionKeyPairData ToPublicKeyEncryptionKeyPairData()
+    {
+        return new PublicKeyEncryptionKeyPairData(
+            wrappedPrivateKey: EncryptedPrivateKey,
+            publicKey: PublicKey);
+    }
 }