Sandbox-Test: Deskriptoren von aussen zaehlen (#654)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/ClawdDotNet.Core.Tests/Sandbox/SandboxEscapeTests.cs

@@ -126,7 +126,9 @@ public sealed class SandboxEscapeTests : IDisposable
     {
         using var held = File.OpenRead(_instanceSecret); // a file the main process has open
 
-        var result = await Sh("for f in /proc/$$/fd/*; do echo ${f##*/}; done");
+        // Listed by a child: a shell globbing its own /proc/$$/fd opens that directory and
+        // would count the handle as fd 3.
+        var result = await Sh("ls /proc/$$/fd");
 
         result.StandardOutput.Split('\n', StringSplitOptions.RemoveEmptyEntries).ShouldBe(["0", "1", "2"], ignoreOrder: true);
     }