Bug: Spawning hydration in response to Idle update (#19011)

* Bug: Spawning hydration in response to Idle update

Adds a test that fails in the new fork.

* Fix typos related to Idle priority

These are just silly mistakes that weren't caught by any of our tests.

There's a lot of duplication in the Lanes module right now. It's also
not super stable as we continue to refine our heuristics. Hopefully the
final state is simpler and less prone to these types of mistakes.