fix(test): make fake timers work with testing-library/react (#25915)

## Summary

Fixes #25869

Two fixes to enable `jest.useFakeTimers()` to work with
`@testing-library/react` and `@testing-library/user-event`:

- Set `setTimeout.clock = true` when fake timers are enabled.
testing-library/react's `jestFakeTimersAreEnabled()` checks for this
property to determine if `jest.advanceTimersByTime()` should be called
when draining the microtask queue. Without this, testing-library never
advances timers.

- Make `advanceTimersByTime(0)` fire `setTimeout(fn, 0)` timers.
`setTimeout(fn, 0)` is internally scheduled with a 1ms delay per HTML
spec. Jest/testing-library expect `advanceTimersByTime(0)` to fire such
"immediate" timers, but we were advancing by 0ms so they never fired.

## Test plan

- [x] All 30 existing fake timer tests pass
- [x] New regression test validates both fixes
- [x] Original user-event reproduction now works (test completes instead
of hanging)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Bot <claude-bot@bun.sh>
Co-authored-by: Claude <noreply@anthropic.com>
Co-authored-by: Jarred Sumner <jarred@jarredsumner.com>

test/regression/issue/25869.test.ts

@@ -0,0 +1,81 @@
+// https://github.com/oven-sh/bun/issues/25869
+// useFakeTimers with testing-library/react hangs when using user-event
+import { expect, jest, test } from "bun:test";
+
+// Test that jestFakeTimersAreEnabled() detection works properly.
+// testing-library/react checks for setTimeout.clock or setTimeout._isMockFunction
+// to determine if fake timers are enabled.
+function jestFakeTimersAreEnabled(): boolean {
+  // @ts-expect-error - checking for Jest fake timers markers
+  if (typeof jest !== "undefined" && jest !== null) {
+    return (
+      // @ts-expect-error - checking for mock function marker
+      (globalThis.setTimeout as any)._isMockFunction === true ||
+      Object.prototype.hasOwnProperty.call(globalThis.setTimeout, "clock")
+    );
+  }
+  return false;
+}
+
+test("setTimeout.clock is not set before useFakeTimers", () => {
+  expect(jestFakeTimersAreEnabled()).toBe(false);
+  expect(Object.prototype.hasOwnProperty.call(globalThis.setTimeout, "clock")).toBe(false);
+});
+
+test("setTimeout.clock is set after useFakeTimers", () => {
+  jest.useFakeTimers();
+  try {
+    expect(jestFakeTimersAreEnabled()).toBe(true);
+    expect(Object.prototype.hasOwnProperty.call(globalThis.setTimeout, "clock")).toBe(true);
+  } finally {
+    jest.useRealTimers();
+  }
+});
+
+test("setTimeout.clock is set to false after useRealTimers", () => {
+  jest.useFakeTimers();
+  jest.useRealTimers();
+  // Note: The clock property remains on setTimeout but is set to false.
+  // This differs from Jest/Sinon which removes the property entirely.
+  // The value being false is sufficient for most use cases.
+  expect((globalThis.setTimeout as any).clock).toBe(false);
+});
+
+test("advanceTimersByTime(0) fires setTimeout(fn, 0) timers", async () => {
+  jest.useFakeTimers();
+  try {
+    let called = false;
+    setTimeout(() => {
+      called = true;
+    }, 0);
+
+    expect(called).toBe(false);
+    jest.advanceTimersByTime(0);
+    expect(called).toBe(true);
+  } finally {
+    jest.useRealTimers();
+  }
+});
+
+test("user-event style wait pattern does not hang", async () => {
+  jest.useFakeTimers();
+  try {
+    // This is the pattern used by @testing-library/user-event in wait.js
+    // It was hanging before the fix because:
+    // 1. advanceTimersByTime(0) didn't fire setTimeout(fn, 0) timers
+    // 2. jestFakeTimersAreEnabled() returned false, so advanceTimers wasn't called
+    const delay = 0;
+
+    const result = await Promise.all([
+      new Promise<string>(resolve => globalThis.setTimeout(() => resolve("timeout"), delay)),
+      Promise.resolve().then(() => {
+        jest.advanceTimersByTime(delay);
+        return "advanced";
+      }),
+    ]);
+
+    expect(result).toEqual(["timeout", "advanced"]);
+  } finally {
+    jest.useRealTimers();
+  }
+});