Use EnvironmentConfigurationSource to read root session ID

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: khanayan123

tracer/src/Datadog.Trace/Util/RuntimeId.cs

@@ -6,6 +6,8 @@
 using System;
 using System.Threading;
 using Datadog.Trace.Configuration;
+using Datadog.Trace.Configuration.ConfigurationSources.Telemetry;
+using Datadog.Trace.Configuration.Telemetry;
 using Datadog.Trace.Logging;
 
 namespace Datadog.Trace.Util
@@ -39,7 +41,8 @@ private static string GetImpl()
 
         private static string GetRootSessionIdImpl()
         {
-            var inherited = EnvironmentHelpers.GetEnvironmentVariable(ConfigurationKeys.Telemetry.RootSessionId);
+            var config = new ConfigurationBuilder(new EnvironmentConfigurationSource(), NullConfigurationTelemetry.Instance);
+            var inherited = config.WithKeys(ConfigurationKeys.Telemetry.RootSessionId).AsString();
             if (!string.IsNullOrEmpty(inherited))
             {
                 Log.Debug("Inherited root session ID from parent: {RootSessionId}", inherited);