Remove revalidation settling period from router-act (#90531)
The 500ms settling period in router-act's `act()` polled for 500ms after
the request queue emptied before exiting. This was added to accommodate
the 300ms revalidation cooldown in the client router — after a server
action calls `revalidatePath`/`revalidateTag`, the client delays
re-prefetching for 300ms. The settling period ensured `act()` waited
long enough to catch the resulting re-prefetch.
The settling period applied to every `act()` call, not just ones
involving revalidation, adding ~500ms per call across 28 test files.
Remove it entirely and instead use Playwright's fake clock
(`page.clock.install` / `page.clock.fastForward`) in the revalidation
tests to deterministically control the cooldown. This is both faster and
architecturally cleaner — the test utility no longer embeds knowledge of
a specific client-side timer.
