Add DbMigrationTest key and update connection logic

Introduced a new static readonly string `DbMigrationTest` in the `Key` class for database migration tests. Updated connection string logic in `Program.cs` to use `DbMigrationTest` when `useDbMigration` is true, allowing for dynamic connection string selection based on environment configuration.

EnvelopeGenerator.Application/Key.cs

@@ -74,4 +74,9 @@ public static class Key
     /// 
     /// </summary>
     public static readonly string Default = nameof(Default);
+
+    /// <summary>
+    /// 
+    /// </summary>
+    public static readonly string DbMigrationTest = nameof(DbMigrationTest);
 }

EnvelopeGenerator.Web/Program.cs

@@ -89,7 +89,9 @@ try
 
     //AddEF Core dbcontext
     var useDbMigration = Environment.GetEnvironmentVariable("MIGRATION_TEST_MODE") == true.ToString() || config.GetValue<bool>("UseDbMigration");
-    var connStr = config.GetConnectionString(Key.Default) ?? throw new InvalidOperationException("There is no default connection string in appsettings.json.");
+    var cnnStrName = useDbMigration ? Key.DbMigrationTest : Key.Default;
+    var connStr = config.GetConnectionString(cnnStrName)
+        ?? throw new InvalidOperationException($"Connection string '{cnnStrName}' is missing in the application configuration.");
 
     builder.Services.AddDistributedSqlServerCache(options =>
     {