Add boolean field to indicate whether or not the Windows thread pool is being used (#90551)

Author: eduardo-vp

src/libraries/System.Private.CoreLib/src/System/Threading/ThreadPool.Windows.cs

@@ -13,6 +13,12 @@ public static partial class ThreadPool
         internal static bool UseWindowsThreadPool { get; } =
             AppContextConfigHelper.GetBooleanConfig("System.Threading.ThreadPool.UseWindowsThreadPool", "DOTNET_ThreadPool_UseWindowsThreadPool");
 
+#pragma warning disable CA1823
+        // The field should reflect what the property returns because the property can be stubbed by trimming,
+        // such that sos reflects the actual state of what thread pool is being used and not just the config value.
+        private static readonly bool s_useWindowsThreadPool = UseWindowsThreadPool; // Name relied on by sos
+#pragma warning restore CA1823
+
 #if NATIVEAOT
         private const bool IsWorkerTrackingEnabledInConfig = false;
 #else