refactor(EnvelopeReceiverQueryBase): simplify EnvelopeReceiverQueryBase.Key handling

- Introduced private backing field `_key` for Key property
- Removed dynamic recomputation from Envelope and Receiver
- Ensured Key is only set once during initialization
- Improved null handling and exception safety

EnvelopeGenerator.Application/Model/EnvelopeReceiverQueryBase.cs

@@ -17,24 +17,26 @@ public record EnvelopeReceiverQueryBase<TEnvelopeQuery, TReceiverQuery>
     where TEnvelopeQuery : EnvelopeQueryBase, new()
     where TReceiverQuery : ReceiverQueryBase, new()
 {
+    private string? _key;
+
     /// <summary>
     /// 
     /// </summary>
     public virtual string? Key
     {
-        get => Envelope?.Uuid is string uuid && Receiver?.Signature is string signature
-                ? (uuid, signature).EncodeEnvelopeReceiverId()
-                : null;
+        get => _key;
         init
         {
             if (value is null)
+            {
+                _key = null;
                 return;
+            }
 
             (string? EnvelopeUuid, string? ReceiverSignature) = value.DecodeEnvelopeReceiverId();
             if (string.IsNullOrEmpty(EnvelopeUuid) || string.IsNullOrEmpty(ReceiverSignature))
-            {
                 throw new BadRequestException("Der EnvelopeReceiverKey muss ein gültiger Base64-kodierter String sein, der die EnvelopeUuid und die ReceiverSignature enthält.");
-            }
+
             Envelope = new TEnvelopeQuery()
             {
                 Uuid = EnvelopeUuid
@@ -43,6 +45,7 @@ public record EnvelopeReceiverQueryBase<TEnvelopeQuery, TReceiverQuery>
             {
                 Signature = ReceiverSignature
             };
+            _key = value;
         }
     }