await-using tests: add microtask-boundary ordering assertions for null/undefined skipped resources

[coderabbitai]

Summary

Add microtask-boundary ordering assertions to the await using null and await using undefined test cases in tests/language/explicit-resource-management/await-using.js to verify that the runtime still introduces an async yield point at block exit even when the disposable resource is skipped.

Why

The current tests only verify that skipped null/undefined values do not throw and that surrounding code continues to run. They do not catch a regression where the compiler/evaluator drops the block-exit await point for skipped disposables, since no ordering assertion against the microtask queue is made.

Current behavior

test("await using null is silently skipped", async () => {
  let reached = false;
  {
    await using resource = null;
    reached = true;
  }
  expect(reached).toBe(true);
});

The test passes but does not verify the presence of the async boundary.

Expected behavior

The tests should use a microtask-ordering pattern to assert the block-exit await point is present even for skipped (null/undefined) resources:

test("await using null is silently skipped", async () => {
  const order = [];
  const task = (async () => {
    {
      await using resource = null;
    }
    order.push("after");
  })();

  expect(order).toEqual([]);
  await Promise.resolve();
  await task;
  expect(order).toEqual(["after"]);
});

Mirror the same pattern for the undefined case.

Scope notes

• File: tests/language/explicit-resource-management/await-using.js
• Only the two null/undefined skipped-resource test cases need updating.
• The underlying runtime behavior (DisposeTrackedResourcesAsync calling AwaitValue for dhAsyncDispose entries even when the method is nil) is already correct per PR Add explicit resource management support #290.
• Backlinks: PR Add explicit resource management support #290, comment Add explicit resource management support #290 (comment)
• Requested by: @frostney