[Link Handling] Links to non-existing content pages should be handled as invalid

currently, the "link handling" logic in the core components works like this:
* if the link URL is a content path pointing to an existing content page, is is externalized (e.g. adding ".html" extension)
* if not the link URL is assumed to be an external URL and inserted unchanged into the markup

actual: in the case that the link URL is a content path (e.g. starting with "/content") that _does not_ point to a valid page (e.g. the target page was deleted or is not yet published), the link handling logic does not detect this and renders the link pointing to an un-externalized and invalid content path.

expected: the link handling should detect invalid internal content page references and handle those links as invalid.

The issue names no implementation files, tests, or entry points. Start by locating the core components' link-handling logic and reproduce a /content link targeting a deleted or unpublished page; done means invalid internal content references are detected and no longer rendered as valid links.

Written by the indexing model from the issue text.