Flag references to whole entries that are unpublished or deleted in delivery reads #2593

Part of #2425 (F-API). A follow-up from the review of #2571.

A resolved view flags references to entity fields that cannot be resolved (unresolved). A reference to a whole entry, however, is passed through as its id even when that entry is unpublished or soft deleted: DeleteAsync does not check incoming references. The delivery JSON then holds an id that /api/v1/entries/{id} answers with 404, and the Markdown holds a link whose .md is a 404.

Scope

• Delivery reads, both JSON and Markdown, flag a whole-entry reference whose target has no published version or is deleted, as unresolved does for entity fields. The flag carries the path and the reason, notPublished or deleted.
• The flag comes from the reference index in the same fixed number of queries.
• In Markdown, such a reference is listed under "Unresolved references" instead of being written as a link.

Acceptance

• A page that references a deleted entry, and one that references an unpublished entry, read with that reference flagged and the reason given. Neither renders a dead link.
• The tests pass on both providers.