Always use private desktops for legacy Windows sandboxes (#46554)

## What changed

- Remove the private-desktop opt-out from elevated and unelevated Windows sandbox launches.
- Remove `windows.sandbox_private_desktop` and its managed requirement and API fields. Warn users to remove the obsolete setting.
- Require a private desktop name when launching through the Windows sandbox wrapper and command runner.

## Testing

Add coverage for the obsolete-setting migration warning and update wrapper tests to verify a live private desktop is passed and a missing desktop name is rejected.

GitOrigin-RevId: c7135f8d211aac8d812180691c2c1e433d77cde4

codex-rs/core/src/tasks/user_shell.rs

@@ -226,10 +226,6 @@ pub(crate) async fn execute_user_shell_command(
         windows_sandbox_policy_cwd: cwd.clone().into(),
         windows_sandbox_workspace_roots: Vec::new(),
         windows_sandbox_level: turn_context.windows_sandbox_level,
-        windows_sandbox_private_desktop: turn_context
-            .config
-            .permissions
-            .windows_sandbox_private_desktop,
         permission_profile,
         windows_sandbox_filesystem_overrides: None,
         arg0: None,