Refactor Envelope properties in Envelope.cs

- Added required `Uuid` property with column mapping.
- Removed `Message` property.
- Changed `Language` from required to nullable string.
- Other properties (`ContractType`, `ExpiresWhen`, `SendReminderEmails`) remain unchanged.

EnvelopeGenerator.Domain/Entities/Envelope.cs

@@ -28,7 +28,6 @@ namespace EnvelopeGenerator.Domain.Entities
         [Column("ENVELOPE_UUID", TypeName = "nvarchar(36)")]
         public required string Uuid { get; init; }
 
-        [Required]
         [Column("MESSAGE", TypeName = "nvarchar(max)")]
         public string? Message { get; set; }
 
@@ -52,7 +51,7 @@ namespace EnvelopeGenerator.Domain.Entities
         public int? ContractType { get; set; }
 
         [Column("LANGUAGE", TypeName = "nvarchar(5)")]
-        public required string Language { get; set; }
+        public string? Language { get; set; }
 
         [Column("SEND_REMINDER_EMAILS")]
         public bool? SendReminderEmails { get; set; }