fix(shells): ref the kill-grace timer so Windows shutdown drain can finish

The second windows-latest hang after the OAuth queue fix had the same
shape: shutdown-drain's tests passed and the isolate process never
exited. The kill-grace wait inside waitForBackgroundShellClose was an
unref'd timer, and Bun on Windows stops servicing unref'd timers when a
pending promise is the only remaining work — so drainAndShutdown waited
on a resolution that never came. The grace timer is now deliberately
ref'd: it self-clears within its two-second bound (or earlier on close),
so it cannot keep a healthy process alive, while the long-lived idle and
absolute deadline timers stay unref'd as before. The test fixture timer
gets the same treatment.

Author: lidge-jun

src/adapters/cursor/native-exec-shell.ts

@@ -340,7 +340,12 @@ function waitForBackgroundShellClose(entry: BackgroundShellEntry): Promise<boole
       settled = true;
       resolveWait(false);
     }, CURSOR_BACKGROUND_SHELL_TERM_GRACE_MS);
-    unrefTimer(timer);
+    // Deliberately REF'D: this is the bounded kill-grace wait that shutdown
+    // drain awaits. Bun on Windows can starve unref'd timers when a pending
+    // promise is the only other work, which would leave drainAndShutdown
+    // waiting on this resolution forever. The timer self-clears within the
+    // 2-second grace window (or earlier on close), so a ref cannot keep the
+    // process alive beyond that bound.
     void entry.closePromise.then(() => {
       if (settled) return;
       settled = true;

tests/shutdown-drain.test.ts

@@ -119,9 +119,13 @@ describe("background shell shutdown drain", () => {
     const unresolvedChild = installShutdownShell();
     setBackgroundShellRuntimeForTests({
       setTimer(callback) {
-        const timer = setTimeout(callback, 0);
-        timer.unref?.();
-        return timer;
+        // Keep this fixture timer REF'D: Bun on Windows can stop servicing
+        // unref'd timers while the test's only pending work is a promise,
+        // which left drainAndShutdown waiting forever and hung the isolate
+        // process until the 20-minute CI job timeout (same starvation the
+        // OAuth queue tests hit). A ref'd 0ms timer fires immediately and
+        // cannot keep the process alive.
+        return setTimeout(callback, 0);
       },
     });
     const unresolvedServer = fakeServer();