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/protocol/src/environment.rs

@@ -69,8 +69,6 @@ pub struct EnvironmentConfig {
     pub windows_sandbox_level: WindowsSandboxLevel,
     /// Concrete Windows sandbox backend selected for this environment attachment.
     pub windows_sandbox_type: SandboxType,
-    /// Whether Windows sandbox processes use a private desktop.
-    pub windows_sandbox_private_desktop: bool,
     /// Whether Linux sandbox processes use the legacy Landlock backend.
     pub use_legacy_landlock: bool,
     /// Additional managed command restrictions for this environment attachment.
@@ -93,10 +91,6 @@ impl std::fmt::Debug for EnvironmentConfig {
             .field("shell_environment_policy", &"<redacted>")
             .field("windows_sandbox_level", &self.windows_sandbox_level)
             .field("windows_sandbox_type", &self.windows_sandbox_type)
-            .field(
-                "windows_sandbox_private_desktop",
-                &self.windows_sandbox_private_desktop,
-            )
             .field("use_legacy_landlock", &self.use_legacy_landlock)
             .field("exec_policy", &self.exec_policy)
             .field("mcp_policy", &self.mcp_policy)