Closes #29788

Stack:

1. DevTools: Rely on sourcemaps to compute hook name of built-in hooks in newer versions #28593
2. Devtools: Add support for useFormStatus #28413
3. [Devtools] Ensure initial read of useFormStatus returns NotPendingTransition #28728 <--- You're here

Summary

We couldn't initialize the HostTransitionContext to NotPendingTransition since that created a dependency cycle. By moving the whole HostTransitionContext into the Fiber config, we no longer have a dependency cycle and can initialize the context to the config specific NotPendingTransition value.

Flow should ensure every Fiber config implements HostTransitionContext properly. It's just a bit annoying since it duplicates config boilerplate.

An incorrect initial value in Devtools would be bad since it probably makes any component not inspectable if you destructure the form status i.e. const {isPending} = useFormStatus() since you'd try to destructure null when we inspect the component.