[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/AdminConsole/Controllers/ProviderClientsController.cs

@@ -57,8 +57,7 @@ public class ProviderClientsController(
             Owner = user,
             BillingEmail = provider.BillingEmail,
             OwnerKey = requestBody.Key,
-            PublicKey = requestBody.KeyPair.PublicKey,
-            PrivateKey = requestBody.KeyPair.EncryptedPrivateKey,
+            Keys = requestBody.KeyPair.ToPublicKeyEncryptionKeyPairData(),
             CollectionName = requestBody.CollectionName,
             IsFromProvider = true
         };