refactor(AsymCryptService): Methode Default.get hinzugefügt, um den ersten Decryptor zu erhalten.

- Wirft InvalidOperationException, wenn kein Decryptor verfügbar ist.

DigitalData.Core.Security/AsymCryptService.cs

@@ -12,6 +12,13 @@ namespace DigitalData.Core.Security
     {
         public IEnumerable<IRSADecryptor> Decryptors => _params.Decryptors;
 
+        public IRSADecryptor Default => Decryptors.FirstOrDefault()
+            ?? throw new InvalidOperationException(
+                "No default decryptor is available. Ensure that at least one decryptor is configured in the provided parameters. " +
+                "This issue typically arises if the configuration for decryptors is incomplete or missing. " +
+                "Check the 'Decryptors' collection in the configuration and verify that it contains valid entries."
+            );
+
         public IRSADecryptor this[string key]
         {
             get