Fix potential race condition in createRouterAct (#78473)
A few of the tests that use `createRouterAct` occasionally flake in CI.
I'm not exactly sure why because I'm struggling to reproduce them
locally, but my best guess based on some of the logs is that there's a
race condition between the IntersectionObserver event that triggers the
prefetch request, and the async event scheduled by `createRouterAct`
that is meant to always run after that.
I had thought `requestAnimationFrame` would be a good way to ensure that
the event runs after the IntersectionObserver, but I think that's not
always correct and depends on the timing of the paint/layout cycle. So
I've switched it to `requestIdleCallback` instead since that won't run
until the rest of the event queue is empty.
(A timer would obviously work here, too, but we should avoid using
timers except to timeout a failing test, because they slow down CI.)
I've also added better error message to `createRouterAct` in the case
where an unexpected request is initiated. The updated message now
includes both the URL and the response body.
