Use a private desktop for Windows sandbox instead of Winsta0\Default (#14400)

## Summary
- launch Windows sandboxed children on a private desktop instead of
`Winsta0\Default`
- make private desktop the default while keeping
`windows.sandbox_private_desktop=false` as the escape hatch
- centralize process launch through the shared
`create_process_as_user(...)` path
- scope the private desktop ACL to the launching logon SID

## Why
Today sandboxed Windows commands run on the visible shared desktop. That
leaves an avoidable same-desktop attack surface for window interaction,
spoofing, and related UI/input issues. This change moves sandboxed
commands onto a dedicated per-launch desktop by default so the sandbox
no longer shares `Winsta0\Default` with the user session.

The implementation stays conservative on security with no silent
fallback back to `Winsta0\Default`

If private-desktop setup fails on a machine, users can still opt out
explicitly with `windows.sandbox_private_desktop=false`.

## Validation
- `cargo build -p codex-cli`
- elevated-path `codex exec` desktop-name probe returned
`CodexSandboxDesktop-*`
- elevated-path `codex exec` smoke sweep for shell commands, nested
`pwsh`, jobs, and hidden `notepad` launch
- unelevated-path full private-desktop compatibility sweep via `codex
exec` with `-c windows.sandbox=unelevated`

codex-rs/app-server/src/command_exec.rs

@@ -733,6 +733,7 @@ mod tests {
             expiration: ExecExpiration::DefaultTimeout,
             sandbox: SandboxType::WindowsRestrictedToken,
             windows_sandbox_level: WindowsSandboxLevel::Disabled,
+            windows_sandbox_private_desktop: false,
             sandbox_permissions: codex_core::sandboxing::SandboxPermissions::UseDefault,
             sandbox_policy: sandbox_policy.clone(),
             file_system_sandbox_policy: FileSystemSandboxPolicy::from(&sandbox_policy),
@@ -844,6 +845,7 @@ mod tests {
                     expiration: ExecExpiration::Cancellation(CancellationToken::new()),
                     sandbox: SandboxType::None,
                     windows_sandbox_level: WindowsSandboxLevel::Disabled,
+                    windows_sandbox_private_desktop: false,
                     sandbox_permissions: codex_core::sandboxing::SandboxPermissions::UseDefault,
                     sandbox_policy: sandbox_policy.clone(),
                     file_system_sandbox_policy: FileSystemSandboxPolicy::from(&sandbox_policy),