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/network-proxy/tests/windows_stable_ingress.rs

@@ -452,9 +452,7 @@ fn run_restricted_child_blocking(
             /*logs_base_dir*/ None,
             /*stdio*/ None,
             /*console_mode*/ ConsoleMode::Inherit,
-            LaunchDesktop::prepare(
-                /*use_private_desktop*/ false, /*logs_base_dir*/ None,
-            )?,
+            LaunchDesktop::prepare(/*logs_base_dir*/ None)?,
         )?
     };
     let process = unsafe {