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/exec/tests/suite/sandbox.rs

@@ -45,7 +45,6 @@ pub(super) async fn spawn_command_under_sandbox(
             network_environment_id: None,
             sandbox_permissions: SandboxPermissions::UseDefault,
             windows_sandbox_level: WindowsSandboxLevel::Disabled,
-            windows_sandbox_private_desktop: false,
             justification: None,
             arg0: None,
         },