fix: keep codex stdio server alive across turns

src/selftest/fake-codex-app-server.ts

@@ -1,7 +1,9 @@
 import * as readline from "node:readline";
+import { appendFileSync } from "node:fs";
 
 const rl = readline.createInterface({ input: process.stdin, crlfDelay: Infinity });
 const mode = process.env.AGENTRUN_FAKE_CODEX_MODE ?? "success";
+if (process.env.AGENTRUN_FAKE_CODEX_START_FILE) appendFileSync(process.env.AGENTRUN_FAKE_CODEX_START_FILE, `${process.pid}\n`);
 let threadCounter = 0;
 let turnCounter = 0;
 let observedThreadModel = false;