Improve Windows process management edge cases (#19211)

## Summary

Some improvements to Windows process-management issues from
https://github.com/openai/codex/pull/15578

- bound the elevated runner pipe-connect handshake instead of waiting
forever on blocking pipe connects
- terminate the spawned runner if that handshake fails, so timeout/error
paths do not leave a stray `codex-command-runner.exe`
- loop on partial `WriteFile` results when forwarding stdin in the
elevated runner, so input is not silently truncated
- fix the concrete HANDLE/SID cleanup paths in the runner setup code
- keep draining driver-backed stdout/stderr after exit until the backend
closes, instead of dropping the tail after a fixed 200ms grace period
- reuse `LocalSid` for SID ownership and add more explanatory comments
around the ownership/concurrency-sensitive code paths

## Why

The original PR fixed a lot of Windows session plumbing, but there were
still a few sharp process-lifecycle edges:

- some elevated runner handshakes could block forever
- the new timeout path could still orphan the spawned runner process
- stdin forwarding still assumed a single `WriteFile` consumed the whole
buffer
- a few raw HANDLE/SID error paths still leaked
- driver-backed output could still lose the last chunk of stdout/stderr
on slower backends

## Validation

- `cargo fmt -p codex-windows-sandbox -p codex-utils-pty`
- `cargo test -p codex-utils-pty`
- `cargo test -p codex-windows-sandbox finish_driver_spawn`
- `cargo test -p codex-windows-sandbox runner_`

Ran a local test matrix of unified-exec and shell_tool tests, all
passing

codex-rs/windows-sandbox-rs/src/unified_exec/backends/elevated.rs

@@ -59,11 +59,13 @@ pub(crate) async fn spawn_windows_sandbox_session_elevated(
     let sandbox_creds = elevated.sandbox_creds.clone();
     let logs_base_dir = elevated.common.logs_base_dir.clone();
     let transport = tokio::task::spawn_blocking(move || -> Result<_> {
-        let mut transport =
-            spawn_runner_transport(&codex_home, &cwd, &sandbox_creds, logs_base_dir.as_deref())?;
-        transport.send_spawn_request(spawn_request)?;
-        transport.read_spawn_ready_with_timeout()?;
-        Ok(transport)
+        spawn_runner_transport(
+            &codex_home,
+            &cwd,
+            &sandbox_creds,
+            logs_base_dir.as_deref(),
+            spawn_request,
+        )
     })
     .await
     .map_err(|err| anyhow::anyhow!("runner handshake task failed: {err}"))??;