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.Domain/Entities/EnvelopeType.cs

@@ -56,7 +56,7 @@ namespace EnvelopeGenerator.Domain.Entities
         public int ReminderIntervalDays { get; set; } = 0;
 
         [Column("CONTRACT_TYPE")]
-        public int ContractType { get; set; }
+        public Constants.ContractType ContractType { get; set; }
 
         [NotMapped]
         public bool TFA_Enabled { get; set; } = false;