Remove DbModelConfigurationException and related logic

Removed the DbModelConfigurationException class and all its usages from DbModelOptions and EntityOptions. Indexers in these classes no longer throw this exception when configuration is missing. Updated the project file to include the Common\Options\DbModel\ folder.

src/ReC.Application/Common/Exceptions/DbModelConfigurationException.cs

@@ -1,8 +0,0 @@
-namespace ReC.Application.Common.Exceptions;
-
-public class DbModelConfigurationException : Exception
-{
-    public DbModelConfigurationException() { }
-
-    public DbModelConfigurationException(string message) : base(message) { }
-}