Improve user agent string construction in Marshaller.cs

Added a `using System.Diagnostics;` directive. Introduced a `retryMode` variable and added a `Debug.Assert` to ensure its correctness. Updated user agent string logic to use `retryMode` and `requestContext.IsAsync`. Changed initialization collections flag to `AWSConfigs.InitializeCollections`.

Author: paulomorgado

sdk/src/Core/Amazon.Runtime/Pipeline/Handlers/Marshaller.cs

@@ -15,6 +15,7 @@
 
 using System;
 using System.Collections.Generic;
+using System.Diagnostics;
 using System.Globalization;
 using System.Text;
 using Amazon.Util;
@@ -114,11 +115,13 @@ private static void SetUserAgentHeader(IRequestContext requestContext)
             if (!string.IsNullOrEmpty(clientAppId))
                 sb.Append(" app/").Append(InternalSDKUtils.ReplaceInvalidUserAgentCharacters(clientAppId));
 
-            sb.Append(" cfg/retry-mode#}").Append(ToUserAgentHeaderString(requestContext.ClientConfig.RetryMode));
+            var retryMode = ToUserAgentHeaderString(requestContext.ClientConfig.RetryMode);
+            Debug.Assert(retryMode != requestContext.ClientConfig.RetryMode.ToString().ToLower(), "Invalid RetryMode string.");
+            sb.Append(" cfg/retry-mode#}").Append(retryMode);
 
-            sb.Append(" md/").Append(IsAsync ? "ClientAsync" : "ClientSync");
+            sb.Append(" md/").Append(requestContext.IsAsync ? "ClientAsync" : "ClientSync");
 
-            sb.Append(" cfg/init-coll#").Append(InitializeCollections ? '1' : '0');
+            sb.Append(" cfg/init-coll#").Append(AWSConfigs.InitializeCollections ? '1' : '0');
 
             var userAgentAddition = requestContext.OriginalRequest.UserAgentAddition;
             if (!string.IsNullOrEmpty(userAgentAddition))