Avoid shell kill cwd cleanup flake

Run the long-lived shell kill command outside the fixture workspace so a surviving Windows child process cannot hold copilot-test-work-* open during cleanup.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: stephentoub

dotnet/test/E2E/RpcShellAndFleetE2ETests.cs

@@ -34,7 +34,9 @@ public async Task Should_Kill_Shell_Process()
             ? "powershell -NoLogo -NoProfile -Command \"Start-Sleep -Seconds 30\""
             : "sleep 30";
 
-        var execResult = await session.Rpc.Shell.ExecAsync(command);
+        // On Windows, terminating the shell wrapper can briefly leave grandchildren alive.
+        // Keep this command outside the fixture workspace so that cleanup is not blocked by cwd handles.
+        var execResult = await session.Rpc.Shell.ExecAsync(command, cwd: Path.GetTempPath());
         Assert.False(string.IsNullOrWhiteSpace(execResult.ProcessId));
 
         var killResult = await session.Rpc.Shell.KillAsync(execResult.ProcessId);