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/windows-sandbox-rs/src/process.rs

@@ -1,3 +1,4 @@
+use crate::desktop::LaunchDesktop;
 use crate::logging;
 use crate::winutil::format_last_error;
 use crate::winutil::quote_windows_arg;
@@ -22,6 +23,12 @@ use windows_sys::Win32::System::Threading::PROCESS_INFORMATION;
 use windows_sys::Win32::System::Threading::STARTF_USESTDHANDLES;
 use windows_sys::Win32::System::Threading::STARTUPINFOW;
 
+pub struct CreatedProcess {
+    pub process_info: PROCESS_INFORMATION,
+    pub startup_info: STARTUPINFOW,
+    _desktop: LaunchDesktop,
+}
+
 pub fn make_env_block(env: &HashMap<String, String>) -> Vec<u16> {
     let mut items: Vec<(String, String)> =
         env.iter().map(|(k, v)| (k.clone(), v.clone())).collect();
@@ -68,7 +75,8 @@ pub unsafe fn create_process_as_user(
     env_map: &HashMap<String, String>,
     logs_base_dir: Option<&Path>,
     stdio: Option<(HANDLE, HANDLE, HANDLE)>,
-) -> Result<(PROCESS_INFORMATION, STARTUPINFOW)> {
+    use_private_desktop: bool,
+) -> Result<CreatedProcess> {
     let cmdline_str = argv
         .iter()
         .map(|a| quote_windows_arg(a))
@@ -80,9 +88,9 @@ pub unsafe fn create_process_as_user(
     si.cb = std::mem::size_of::<STARTUPINFOW>() as u32;
     // Some processes (e.g., PowerShell) can fail with STATUS_DLL_INIT_FAILED
     // if lpDesktop is not set when launching with a restricted token.
-    // Point explicitly at the interactive desktop.
-    let desktop = to_wide("Winsta0\\Default");
-    si.lpDesktop = desktop.as_ptr() as *mut u16;
+    // Point explicitly at the interactive desktop or a private desktop.
+    let desktop = LaunchDesktop::prepare(use_private_desktop, logs_base_dir)?;
+    si.lpDesktop = desktop.startup_info_desktop();
     let mut pi: PROCESS_INFORMATION = std::mem::zeroed();
     // Ensure handles are inheritable when custom stdio is supplied.
     let inherit_handles = match stdio {
@@ -107,6 +115,10 @@ pub unsafe fn create_process_as_user(
         }
     };
 
+    let creation_flags = CREATE_UNICODE_ENVIRONMENT;
+    let cwd_wide = to_wide(cwd);
+    let env_block_len = env_block.len();
+
     let ok = CreateProcessAsUserW(
         h_token,
         std::ptr::null(),
@@ -114,25 +126,30 @@ pub unsafe fn create_process_as_user(
         std::ptr::null_mut(),
         std::ptr::null_mut(),
         inherit_handles as i32,
-        CREATE_UNICODE_ENVIRONMENT,
+        creation_flags,
         env_block.as_ptr() as *mut c_void,
-        to_wide(cwd).as_ptr(),
+        cwd_wide.as_ptr(),
         &si,
         &mut pi,
     );
     if ok == 0 {
         let err = GetLastError() as i32;
         let msg = format!(
-            "CreateProcessAsUserW failed: {} ({}) | cwd={} | cmd={} | env_u16_len={} | si_flags={}",
+            "CreateProcessAsUserW failed: {} ({}) | cwd={} | cmd={} | env_u16_len={} | si_flags={} | creation_flags={}",
             err,
             format_last_error(err),
             cwd.display(),
             cmdline_str,
-            env_block.len(),
+            env_block_len,
             si.dwFlags,
+            creation_flags,
         );
         logging::debug_log(&msg, logs_base_dir);
         return Err(anyhow!("CreateProcessAsUserW failed: {}", err));
     }
-    Ok((pi, si))
+    Ok(CreatedProcess {
+        process_info: pi,
+        startup_info: si,
+        _desktop: desktop,
+    })
 }