fix: handle early codex exec exit (#8825)

Fixes CodexExec to avoid missing early process exits by registering the
exit handler up front and deferring the error until after stdout is
drained, and adds a regression test that simulates a fast-exit child
while still producing output so hangs are caught.

sdk/typescript/tests/exec.test.ts

@@ -0,0 +1,70 @@
+import * as child_process from "node:child_process";
+import { EventEmitter } from "node:events";
+import { PassThrough } from "node:stream";
+
+import { describe, expect, it } from "@jest/globals";
+
+jest.mock("node:child_process", () => {
+  const actual = jest.requireActual<typeof import("node:child_process")>("node:child_process");
+  return { ...actual, spawn: jest.fn() };
+});
+
+const _actualChildProcess =
+  jest.requireActual<typeof import("node:child_process")>("node:child_process");
+const spawnMock = child_process.spawn as jest.MockedFunction<typeof _actualChildProcess.spawn>;
+
+class FakeChildProcess extends EventEmitter {
+  stdin = new PassThrough();
+  stdout = new PassThrough();
+  stderr = new PassThrough();
+  killed = false;
+
+  kill(): boolean {
+    this.killed = true;
+    return true;
+  }
+}
+
+function createEarlyExitChild(exitCode = 2): FakeChildProcess {
+  const child = new FakeChildProcess();
+  setImmediate(() => {
+    child.stderr.write("boom");
+    child.emit("exit", exitCode, null);
+    setImmediate(() => {
+      child.stdout.end();
+      child.stderr.end();
+    });
+  });
+  return child;
+}
+
+const delay = (ms: number) => new Promise((resolve) => setTimeout(resolve, ms));
+
+describe("CodexExec", () => {
+  it("rejects when exit happens before stdout closes", async () => {
+    const { CodexExec } = await import("../src/exec");
+    const child = createEarlyExitChild();
+    spawnMock.mockReturnValue(child as unknown as child_process.ChildProcess);
+
+    const exec = new CodexExec("codex");
+    const runPromise = (async () => {
+      for await (const _ of exec.run({ input: "hi" })) {
+        // no-op
+      }
+    })().then(
+      () => ({ status: "resolved" as const }),
+      (error) => ({ status: "rejected" as const, error }),
+    );
+
+    const result = await Promise.race([
+      runPromise,
+      delay(500).then(() => ({ status: "timeout" as const })),
+    ]);
+
+    expect(result.status).toBe("rejected");
+    if (result.status === "rejected") {
+      expect(result.error).toBeInstanceOf(Error);
+      expect(result.error.message).toMatch(/Codex Exec exited/);
+    }
+  });
+});