…data w/ parallel routes (#65102)

### What
When a page throws a `notFound()` error in `generateMetadata`, and the
page contains parallel route(s), an unhandled runtime error would be
thrown rather than displaying the not found page.

### Why
We use the `<MetadataOutlet />` component to throw any errors caught
during metadata resolution once the metadata is rendered by React so
that it can be caught by an error boundary, so that it doesn't throw
during metadata resolution. A promise is tracked & resolved with an
error once the metadata tree is rendered. Once the promise resolves, the
outlet component will throw.

However, every `__PAGE__` segment that would be rendered as part of the
page the user is on will render this `<MetadataOutlet />` component. We
only need a single outlet per segment as only a single error needs to be
thrown & caught.

### How
This will only render a `MetadataOutlet` for the first parallel route
that is encountered at each segment depth, as we only need a single
handler to throw the error.

Fixes #65013
Closes NEXT-3222