Add Windows sandbox unified exec runtime support (#15578)

## Summary

This is the runtime/foundation half of the Windows sandbox unified-exec
work.

- add Windows sandbox `unified_exec` session support in
`windows-sandbox-rs` for both:
  - the legacy restricted-token backend
  - the elevated runner backend
- extend the PTY/process runtime so driver-backed sessions can support:
  - stdin streaming
  - stdout/stderr separation
  - exit propagation
  - PTY resize hooks
- add Windows sandbox runtime coverage in `codex-windows-sandbox` /
`codex-utils-pty`

This PR does **not** enable Windows sandbox `UnifiedExec` for product
callers yet because hooking this up to app-server comes in the next PR.

Windows sandbox advertising is intentionally kept aligned with `main`,
so sandboxed Windows callers still fall back to `ShellCommand`.

This PR isolates the runtime/session layer so it can be reviewed
independently from product-surface enablement.

---------

Co-authored-by: jif-oai <jif@openai.com>
Co-authored-by: Codex <noreply@openai.com>

codex-rs/core/src/exec.rs

@@ -314,7 +314,11 @@ pub fn build_exec_request(
             windows_sandbox_level,
             windows_sandbox_private_desktop,
         })
-        .map(|request| ExecRequest::from_sandbox_exec_request(request, options))
+        .map(|request| {
+            let windows_sandbox_policy_cwd = AbsolutePathBuf::try_from(sandbox_cwd.to_path_buf())
+                .unwrap_or_else(|_| request.cwd.clone());
+            ExecRequest::from_sandbox_exec_request(request, options, windows_sandbox_policy_cwd)
+        })
         .map_err(CodexErr::from)?;
     let use_windows_elevated_backend = windows_sandbox_uses_elevated_backend(
         exec_req.windows_sandbox_level,
@@ -357,6 +361,7 @@ pub(crate) async fn execute_exec_request(
         expiration,
         capture_policy,
         sandbox,
+        windows_sandbox_policy_cwd: _,
         windows_sandbox_level,
         windows_sandbox_private_desktop,
         sandbox_policy,