feat(TestBase): seed real email templates in TestBase setup

- Added `RealEmailTemplates` property with predefined email templates.
- Populated the repository with these templates in the `Setup` method.
- Ensures tests have consistent, realistic email data available.

EnvelopeGenerator.Domain/Entities/EmailTemplate.cs

@@ -1,4 +1,5 @@
-using System.ComponentModel;
+using DigitalData.Core.Abstractions.Interfaces;
+using System.ComponentModel;
 using System.ComponentModel.DataAnnotations;
 using System.ComponentModel.DataAnnotations.Schema;
 #if NETFRAMEWORK
@@ -13,7 +14,7 @@ namespace EnvelopeGenerator.Domain.Entities
 #endif
 
 [Table("TBSIG_EMAIL_TEMPLATE", Schema = "dbo")]
-public class EmailTemplate
+public class EmailTemplate : IEntity
 {
     [Key]
     [DatabaseGenerated(DatabaseGeneratedOption.Identity)]