Refactor: centralize HTTP client naming with Constants

Introduced a Constants class to define a unique HttpClientName for HTTP client registration and usage. Updated DependencyInjection and InvokeRecActionCommandHandler to use this centralized name, improving consistency and reducing risk of name collisions.

src/ReC.Application/RecActions/Commands/InvokeRecActionCommand.cs

@@ -31,7 +31,7 @@ public class InvokeRecActionCommandHandler(
     {
         var action = request.Action;
 
-        using var http = clientFactory.CreateClient("Default");
+        using var http = clientFactory.CreateClient(Constants.HttpClientName);
 
         if (action.RestType is null)
             throw new DataIntegrityException(