[DevTools] Don't measure fallbacks when suspended (#34850)

We already do this in the update pass. That's what
`shouldMeasureSuspenseNode` does.

We also don't update measurements when we're inside an offscreen tree.

However, we didn't check if the boundary itself was in a suspended state
when in the `measureUnchangedSuspenseNodesRecursively` path.

This caused boundaries to disappear when their fallback didn't have a
rect (including their timeline entries).