Refactor test DB config to support SQL Server and fix seeding

Refactored Fake.cs to configure both in-memory and SQL Server
database contexts for testing, using the "Default" connection
string from configuration. Added detailed EF logging and SQL
executor setup. In TestBase.cs, fixed Setup to use the correct
repository instance for seeding email templates.

EnvelopeGenerator.Tests/TestBase.cs

@@ -32,9 +32,11 @@ public abstract class TestBase : Faker
         Host = Fake.CreateHost(ConfigureServices);
         await Host.AddSamples();
 
+        var repo = GetRepository<EmailTemplate>();
+
         // Add seed email templates
         foreach (var temp in SeedEmailTemplates)
-            await Repository.CreateAsync(temp);
+            await repo.CreateAsync(temp);
     }
 
     [TearDown]