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/shell_snapshot_tests.rs

@@ -301,7 +301,6 @@ async fn inactive_profiles_keep_snapshots_but_active_brokers_require_sandbox() -
         workspace_roots: Vec::new(),
         windows_sandbox_level: WindowsSandboxLevel::Disabled,
         windows_sandbox_type: SandboxType::None,
-        windows_sandbox_private_desktop: true,
         use_legacy_landlock: false,
         permission_profile: PermissionProfileSnapshot::legacy(permission_profile),
         shell_environment_policy: ShellEnvironmentPolicy::default(),
@@ -423,7 +422,6 @@ async fn inactive_profiles_keep_snapshots_but_active_brokers_require_sandbox() -
         use_legacy_landlock: false,
         windows_sandbox_type: SandboxType::None,
         windows_sandbox_level: WindowsSandboxLevel::Disabled,
-        windows_sandbox_private_desktop: false,
         network_denial_cancellation_token: Some(cancellation.clone()),
         network_proxy: None,
     };