[dynamicIO] Disallow only dynamic metadata (#78576)
stacked on #78575
When I changed the dynamic validation rules to be based on the existence
of a static shell I removed an important protection for apps that have
static metadata. Now that metadata is implicitly rendered within a
Suspense boundary it is always opted into allowing dynamic. For dynamic
and partially static pages this is fine because we are going to be
generating a response per request anyways. But if you have a fully
static page and then later accidentally make your metadata dynamic your
page will deopt to partially static without any warning.
This change reintroduces the heuristic where if the only dynamic thing
on the page is metadata the build errors. If there is at least one other
dynamic thing on the page then dynamic metadata is allowed.
A similar change is not necessary for viewport because that is never
rendered in a Suspense boundary and the only way to have dynamic
viewports is to opt the entire app into dynamic with a Suspense boundary
around your root layout
