Use a private desktop for Windows sandbox instead of Winsta0\Default (#14400)

## Summary
- launch Windows sandboxed children on a private desktop instead of
`Winsta0\Default`
- make private desktop the default while keeping
`windows.sandbox_private_desktop=false` as the escape hatch
- centralize process launch through the shared
`create_process_as_user(...)` path
- scope the private desktop ACL to the launching logon SID

## Why
Today sandboxed Windows commands run on the visible shared desktop. That
leaves an avoidable same-desktop attack surface for window interaction,
spoofing, and related UI/input issues. This change moves sandboxed
commands onto a dedicated per-launch desktop by default so the sandbox
no longer shares `Winsta0\Default` with the user session.

The implementation stays conservative on security with no silent
fallback back to `Winsta0\Default`

If private-desktop setup fails on a machine, users can still opt out
explicitly with `windows.sandbox_private_desktop=false`.

## Validation
- `cargo build -p codex-cli`
- elevated-path `codex exec` desktop-name probe returned
`CodexSandboxDesktop-*`
- elevated-path `codex exec` smoke sweep for shell commands, nested
`pwsh`, jobs, and hidden `notepad` launch
- unelevated-path full private-desktop compatibility sweep via `codex
exec` with `-c windows.sandbox=unelevated`

codex-rs/core/src/windows_sandbox_tests.rs

@@ -77,12 +77,14 @@ fn resolve_windows_sandbox_mode_prefers_profile_windows() {
     let cfg = ConfigToml {
         windows: Some(WindowsToml {
             sandbox: Some(WindowsSandboxModeToml::Unelevated),
+            ..Default::default()
         }),
         ..Default::default()
     };
     let profile = ConfigProfile {
         windows: Some(WindowsToml {
             sandbox: Some(WindowsSandboxModeToml::Elevated),
+            ..Default::default()
         }),
         ..Default::default()
     };
@@ -130,3 +132,47 @@ fn resolve_windows_sandbox_mode_profile_legacy_false_blocks_top_level_legacy_tru
 
     assert_eq!(resolve_windows_sandbox_mode(&cfg, &profile), None);
 }
+
+#[test]
+fn resolve_windows_sandbox_private_desktop_prefers_profile_windows() {
+    let cfg = ConfigToml {
+        windows: Some(WindowsToml {
+            sandbox: Some(WindowsSandboxModeToml::Unelevated),
+            sandbox_private_desktop: Some(false),
+        }),
+        ..Default::default()
+    };
+    let profile = ConfigProfile {
+        windows: Some(WindowsToml {
+            sandbox: Some(WindowsSandboxModeToml::Elevated),
+            sandbox_private_desktop: Some(true),
+        }),
+        ..Default::default()
+    };
+
+    assert!(resolve_windows_sandbox_private_desktop(&cfg, &profile));
+}
+
+#[test]
+fn resolve_windows_sandbox_private_desktop_defaults_to_true() {
+    assert!(resolve_windows_sandbox_private_desktop(
+        &ConfigToml::default(),
+        &ConfigProfile::default()
+    ));
+}
+
+#[test]
+fn resolve_windows_sandbox_private_desktop_respects_explicit_cfg_value() {
+    let cfg = ConfigToml {
+        windows: Some(WindowsToml {
+            sandbox_private_desktop: Some(false),
+            ..Default::default()
+        }),
+        ..Default::default()
+    };
+
+    assert!(!resolve_windows_sandbox_private_desktop(
+        &cfg,
+        &ConfigProfile::default()
+    ));
+}