Refactor envelope type handling and validation logic

- Change ContractType property to use Constants.ContractType enum for improved type safety.
- Move EnvelopeType selection to a shared module-level variable for consistent access.
- Allow user selection of contract type when creating envelopes.
- Refine envelope validation to check for required elements per receiver, with improved error messages.

EnvelopeGenerator.Form/Controllers/EnvelopeEditorController.vb

@@ -50,17 +50,21 @@ Public Class EnvelopeEditorController
     Public Function ValidateEnvelopeForSending(pErrors As List(Of String)) As List(Of String)
         Dim oEnvelopeErrors = pErrors
 
-        If ElementModel.ElementsExist(Envelope.Id) = False Then
-            oEnvelopeErrors.Add(Resources.Envelope.Missing_Elements)
-        End If
+        If Envelope.EnvelopeTypeId <> Domain.Constants.ContractType.ReadAndSign Then
 
-        If ElementModel.OneElementPerReceiverExist(Envelope.Id) = False Then
+            If ElementModel.ElementsExist(Envelope.Id) = False Then
+                oEnvelopeErrors.Add(Resources.Envelope.Missing_Elements)
+            End If
 
-            For Each receiverItem As Receiver In Envelope.Receivers
-                If ElementModel.ElementsExist(Envelope.Id, receiverItem.Id) = False Then
-                    oEnvelopeErrors.Add(String.Format(Resources.Envelope.Missing_Elements_for_Receiver, receiverItem.Name))
-                End If
-            Next
+            If ElementModel.OneElementPerReceiverExist(Envelope.Id) = False Then
+
+                For Each receiverItem As Receiver In Envelope.Receivers
+                    If ElementModel.ElementsExist(Envelope.Id, receiverItem.Id) = False Then
+                        oEnvelopeErrors.Add(String.Format(Resources.Envelope.Missing_Elements_for_Receiver, receiverItem.Name))
+                    End If
+                Next
+
+            End If
 
         End If