Fix: Rules of Hooks violation in AppRouter (#80623)
There's some sketchy logic in AppRouter that (intentionally) violates
the rules of hooks by reading an externally mutable condition. The
rationalization is that it's OK to do this if the result is to
infinitely suspend the component.
However, the mechanism used to infinitely suspend the component is
`use(infinitePromise)` — this can lead to subtle bugs because even
though `use` can be called conditionally, it relies on the assumption
that its invocation is idempotent for the same props and state, i.e. you
must still follow the rules of React regarding mutability and side
effects.
Ideally we should refactor this block to not rely on external state at
all, but in the meantime, we can switch from `use` to `throw` to suspend
the component, since `throw` is lower level and isn't modeled as a hook.
