Follow-on to the non-dictionary /Font and /XObject guard. Replace the
_dict_entries() helper with an isinstance(..., pikepdf.Dictionary) check
at each resource lookup, which pikepdf's metaclass supports directly and
which reads as exactly the invariant being enforced. Iterate via
as_dict().values() so the values are typed and mypy stays clean once the
Any from the untyped resources argument is narrowed away.
Also guard _cid_font_is_embedded against a non-dictionary /FontDescriptor:
`key in descriptor` raises ValueError on a non-dict, which the caller's
except (AttributeError, TypeError, KeyError) does not catch. Such a font
now counts as non-embedded and is reported, so PDF/A conversion is refused
rather than risking Ghostscript corrupting a pre-existing CID text layer.
Adds a regression test for the FontDescriptor case (issue #1713).
James R. Barlow
640b3062b2