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 +6,7 @@
 using System;
 using System.Threading;
 using Datadog.Trace.Configuration;
+using Datadog.Trace.Configuration.Telemetry;
 using Datadog.Trace.Logging;
 
 namespace Datadog.Trace.Util
@@ -39,7 +40,9 @@ private static string GetImpl()
 
         private static string GetRootSessionIdImpl()
         {
-            var inherited = EnvironmentHelpers.GetEnvironmentVariable(ConfigurationKeys.Telemetry.RootSessionId);
+            var source = new EnvironmentConfigurationSource();
+            var result = source.GetString(ConfigurationKeys.Telemetry.RootSessionId, NullConfigurationTelemetry.Instance, validator: null, recordValue: true);
+            var inherited = result.IsValid ? result.Result : null;
             if (!string.IsNullOrEmpty(inherited))
             {
                 Log.Debug("Inherited root session ID from parent: {RootSessionId}", inherited);