Updated DesktopOsHelper.IsMac to work properly on net10 + macOS 26

Author: reynoldskr

src/client/Microsoft.Identity.Client/PlatformsCommon/Shared/DesktopOsHelper.cs

@@ -53,7 +53,10 @@ public static bool IsMac()
             return Environment.OSVersion.Platform == PlatformID.MacOSX;
 #elif NET8_0_OR_GREATER
             string OSDescription = RuntimeInformation.OSDescription;
-            return OSDescription.Contains("Darwin", StringComparison.OrdinalIgnoreCase);
+
+            // .NET 10 and later changes the OS string in `OSDescription`
+            return OSDescription.Contains("Darwin", StringComparison.OrdinalIgnoreCase) ||
+                   OSDescription.Contains("macOS", StringComparison.OrdinalIgnoreCase);
 #else
             return RuntimeInformation.IsOSPlatform(OSPlatform.OSX);
 #endif