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/Common/Constants.cs

@@ -0,0 +1,6 @@
+namespace ReC.Application.Common;
+
+public static class Constants
+{
+    public static readonly string HttpClientName = "HttpClient-" + Guid.NewGuid().ToString();
+}