windows-sandbox: add runner IPC foundation for future unified_exec (#14139)

# Summary

This PR introduces the Windows sandbox runner IPC foundation that later
unified_exec work will build on.

The key point is that this is intentionally infrastructure-only. The new
IPC transport, runner plumbing, and ConPTY helpers are added here, but
the active elevated Windows sandbox path still uses the existing
request-file bootstrap. In other words, this change prepares the
transport and module layout we need for unified_exec without switching
production behavior over yet.

Part of this PR is also a source-layout cleanup: some Windows sandbox
files are moved into more explicit `elevated/`, `conpty/`, and shared
locations so it is clearer which code is for the elevated sandbox flow,
which code is legacy/direct-spawn behavior, and which helpers are shared
between them. That reorganization is intentional in this first PR so
later behavioral changes do not also have to carry a large amount of
file-move churn.

# Why This Is Needed For unified_exec

Windows elevated sandboxed unified_exec needs a long-lived,
bidirectional control channel between the CLI and a helper process
running under the sandbox user. That channel has to support:

- starting a process and reporting structured spawn success/failure
- streaming stdout/stderr back incrementally
- forwarding stdin over time
- terminating or polling a long-lived process
- supporting both pipe-backed and PTY-backed sessions

The existing elevated one-shot path is built around a request-file
bootstrap and does not provide those primitives cleanly. Before we can
turn on Windows sandbox unified_exec, we need the underlying runner
protocol and transport layer that can carry those lifecycle events and
streams.

# Why Windows Needs More Machinery Than Linux Or macOS

Linux and macOS can generally build unified_exec on top of the existing
sandbox/process model: the parent can spawn the child directly, retain
normal ownership of stdio or PTY handles, and manage the lifetime of the
sandboxed process without introducing a second control process.

Windows elevated sandboxing is different. To run inside the sandbox
boundary, we cross into a different user/security context and then need
to manage a long-lived process from outside that boundary. That means we
need an explicit helper process plus an IPC transport to carry spawn,
stdin, output, and exit events back and forth. The extra code here is
mostly that missing Windows sandbox infrastructure, not a conceptual
difference in unified_exec itself.

# What This PR Adds

- the framed IPC message types and transport helpers for parent <->
runner communication
- the renamed Windows command runner with both the existing request-file
bootstrap and the dormant IPC bootstrap
- named-pipe helpers for the elevated runner path
- ConPTY helpers and process-thread attribute plumbing needed for
PTY-backed sessions
- shared sandbox/process helpers that later PRs will reuse when
switching live execution paths over
- early file/module moves so later PRs can focus on behavior rather than
layout churn

# What This PR Does Not Yet Do

- it does not switch the active elevated one-shot path over to IPC yet
- it does not enable Windows sandbox unified_exec yet
- it does not remove the existing request-file bootstrap yet

So while this code compiles and the new path has basic validation, it is
not yet the exercised production path. That is intentional for this
first PR: the goal here is to land the transport and runner foundation
cleanly before later PRs start routing real command execution through
it.

# Follow-Ups

Planned follow-up PRs will:

1. switch elevated one-shot Windows sandbox execution to the new runner
IPC path
2. layer Windows sandbox unified_exec sessions on top of the same
transport
3. remove the legacy request-file path once the IPC-based path is live

# Validation

- `cargo build -p codex-windows-sandbox`

codex-rs/windows-sandbox-rs/src/conpty/mod.rs

@@ -0,0 +1,139 @@
+//! ConPTY helpers for spawning sandboxed processes with a PTY on Windows.
+//!
+//! This module encapsulates ConPTY creation and process spawn with the required
+//! `PROC_THREAD_ATTRIBUTE_PSEUDOCONSOLE` plumbing. It is shared by both the legacy
+//! restricted‑token path and the elevated runner path when unified_exec runs with
+//! `tty=true`. The helpers are not tied to the IPC layer and can be reused by other
+//! Windows sandbox flows that need a PTY.
+
+mod proc_thread_attr;
+
+use self::proc_thread_attr::ProcThreadAttributeList;
+use crate::winutil::format_last_error;
+use crate::winutil::quote_windows_arg;
+use crate::winutil::to_wide;
+use anyhow::Result;
+use codex_utils_pty::RawConPty;
+use std::collections::HashMap;
+use std::ffi::c_void;
+use std::path::Path;
+use windows_sys::Win32::Foundation::CloseHandle;
+use windows_sys::Win32::Foundation::GetLastError;
+use windows_sys::Win32::Foundation::HANDLE;
+use windows_sys::Win32::Foundation::INVALID_HANDLE_VALUE;
+use windows_sys::Win32::System::Console::ClosePseudoConsole;
+use windows_sys::Win32::System::Threading::CreateProcessAsUserW;
+use windows_sys::Win32::System::Threading::CREATE_UNICODE_ENVIRONMENT;
+use windows_sys::Win32::System::Threading::EXTENDED_STARTUPINFO_PRESENT;
+use windows_sys::Win32::System::Threading::PROCESS_INFORMATION;
+use windows_sys::Win32::System::Threading::STARTF_USESTDHANDLES;
+use windows_sys::Win32::System::Threading::STARTUPINFOEXW;
+
+use crate::process::make_env_block;
+
+/// Owns a ConPTY handle and its backing pipe handles.
+pub struct ConptyInstance {
+    pub hpc: HANDLE,
+    pub input_write: HANDLE,
+    pub output_read: HANDLE,
+}
+
+impl Drop for ConptyInstance {
+    fn drop(&mut self) {
+        unsafe {
+            if self.input_write != 0 && self.input_write != INVALID_HANDLE_VALUE {
+                CloseHandle(self.input_write);
+            }
+            if self.output_read != 0 && self.output_read != INVALID_HANDLE_VALUE {
+                CloseHandle(self.output_read);
+            }
+            if self.hpc != 0 && self.hpc != INVALID_HANDLE_VALUE {
+                ClosePseudoConsole(self.hpc);
+            }
+        }
+    }
+}
+
+impl ConptyInstance {
+    /// Consume the instance and return raw handles without closing them.
+    pub fn into_raw(self) -> (HANDLE, HANDLE, HANDLE) {
+        let me = std::mem::ManuallyDrop::new(self);
+        (me.hpc, me.input_write, me.output_read)
+    }
+}
+
+/// Create a ConPTY with backing pipes.
+///
+/// This is public so callers that need lower-level PTY setup can build on the same
+/// primitive, although the common entry point is `spawn_conpty_process_as_user`.
+pub fn create_conpty(cols: i16, rows: i16) -> Result<ConptyInstance> {
+    let raw = RawConPty::new(cols, rows)?;
+    let (hpc, input_write, output_read) = raw.into_raw_handles();
+
+    Ok(ConptyInstance {
+        hpc: hpc as HANDLE,
+        input_write: input_write as HANDLE,
+        output_read: output_read as HANDLE,
+    })
+}
+
+/// Spawn a process under `h_token` with ConPTY attached.
+///
+/// This is the main shared ConPTY entry point and is used by both the legacy/direct path
+/// and the elevated runner path whenever a PTY-backed sandboxed process is needed.
+pub fn spawn_conpty_process_as_user(
+    h_token: HANDLE,
+    argv: &[String],
+    cwd: &Path,
+    env_map: &HashMap<String, String>,
+) -> Result<(PROCESS_INFORMATION, ConptyInstance)> {
+    let cmdline_str = argv
+        .iter()
+        .map(|arg| quote_windows_arg(arg))
+        .collect::<Vec<_>>()
+        .join(" ");
+    let mut cmdline: Vec<u16> = to_wide(&cmdline_str);
+    let env_block = make_env_block(env_map);
+    let mut si: STARTUPINFOEXW = unsafe { std::mem::zeroed() };
+    si.StartupInfo.cb = std::mem::size_of::<STARTUPINFOEXW>() as u32;
+    si.StartupInfo.dwFlags = STARTF_USESTDHANDLES;
+    si.StartupInfo.hStdInput = INVALID_HANDLE_VALUE;
+    si.StartupInfo.hStdOutput = INVALID_HANDLE_VALUE;
+    si.StartupInfo.hStdError = INVALID_HANDLE_VALUE;
+    let desktop = to_wide("Winsta0\\Default");
+    si.StartupInfo.lpDesktop = desktop.as_ptr() as *mut u16;
+
+    let conpty = create_conpty(80, 24)?;
+    let mut attrs = ProcThreadAttributeList::new(1)?;
+    attrs.set_pseudoconsole(conpty.hpc)?;
+    si.lpAttributeList = attrs.as_mut_ptr();
+
+    let mut pi: PROCESS_INFORMATION = unsafe { std::mem::zeroed() };
+    let ok = unsafe {
+        CreateProcessAsUserW(
+            h_token,
+            std::ptr::null(),
+            cmdline.as_mut_ptr(),
+            std::ptr::null_mut(),
+            std::ptr::null_mut(),
+            0,
+            EXTENDED_STARTUPINFO_PRESENT | CREATE_UNICODE_ENVIRONMENT,
+            env_block.as_ptr() as *mut c_void,
+            to_wide(cwd).as_ptr(),
+            &si.StartupInfo,
+            &mut pi,
+        )
+    };
+    if ok == 0 {
+        let err = unsafe { GetLastError() } as i32;
+        return Err(anyhow::anyhow!(
+            "CreateProcessAsUserW failed: {} ({}) | cwd={} | cmd={} | env_u16_len={}",
+            err,
+            format_last_error(err),
+            cwd.display(),
+            cmdline_str,
+            env_block.len()
+        ));
+    }
+    Ok((pi, conpty))
+}