Enhance framework compatibility and code readability

Added preprocessor directives for .NET framework compatibility.
Modified `using` directives to be framework-specific.
Improved code formatting for better readability.
Introduced obsolete attributes for deprecated methods,
recommending `MediatR` as an alternative.
Added XML documentation for clarity and maintainability.

DigitalData.Core.Abstraction.Application/DTO/Flag.cs

@@ -1,4 +1,5 @@
-namespace DigitalData.Core.Abstraction.Application.DTO;
+#if NET
+namespace DigitalData.Core.Abstraction.Application.DTO;
 
 /// <summary>
 /// Defines flags that indicate specific types of status or conditions in a service operation.
@@ -47,4 +48,5 @@ public enum Flag
     /// This flag is used when the specified item or condition does not exist or is unavailable.
     /// </summary>
     NotFound
-}
+}
+#endif