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/session/mod.rs

@@ -841,7 +841,6 @@ impl Session {
             shell_environment_policy: config.permissions.shell_environment_policy.clone(),
             windows_sandbox_level: WindowsSandboxLevel::from_config(&config),
             windows_sandbox_type: config.permissions.windows_sandbox_type,
-            windows_sandbox_private_desktop: config.permissions.windows_sandbox_private_desktop,
             use_legacy_landlock: config.features.use_legacy_landlock(),
             legacy_fallback_cwd: config.cwd.clone(),
             runtime_workspace_roots: config.workspace_roots.clone(),