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/tests/suite/unified_exec.rs

@@ -506,11 +506,6 @@ async fn exec_command_uses_installed_environment_shell_policy_with_explicit_over
                 },
                 windows_sandbox_level: WindowsSandboxLevel::from_config(&harness.test().config),
                 windows_sandbox_type: harness.test().config.permissions.windows_sandbox_type,
-                windows_sandbox_private_desktop: harness
-                    .test()
-                    .config
-                    .permissions
-                    .windows_sandbox_private_desktop,
                 use_legacy_landlock: harness.test().config.features.use_legacy_landlock(),
                 exec_policy: None,
                 mcp_policy: None,