DevTools: Restore inspect-element bridge optimizations (#20789)

* Restore inspect-element bridge optimizations

When the new Suspense cache was integrated (so that startTransition could be used) I removed a couple of optimizations between the backend and frontend that reduced bridge traffic when e.g. dehydrated paths were inspected for elements that had not rendered since previously inspected. This commit re-adds those optimizations as well as an additional test with a bug fix that I noticed while reading the backend code.

There are two remaining TODO items as of this commit:
- Make inspected element edits and deletes also use transition API
- Don't over-eagerly refresh the cache in our ping-for-updates handler

I will addres both in subsequent commits.

* Poll for update only refreshes cache when there's an update

* Added inline comment