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/cli/src/debug_sandbox.rs

@@ -496,7 +496,6 @@ async fn run_command_under_sandbox(
                 sandbox_exe: Some(codex_self_exe.as_path()),
                 use_legacy_landlock: false,
                 windows_sandbox_level: codex_protocol::config_types::WindowsSandboxLevel::Disabled,
-                windows_sandbox_private_desktop: false,
             })?;
             let (program, args) = request
                 .command
@@ -587,7 +586,6 @@ async fn run_command_under_windows_session(
         deny_write_paths_override: empty_paths,
         tty: false,
         stdin_open: true,
-        use_private_desktop: config.permissions.windows_sandbox_private_desktop,
     })
     .await;