Fix json-insert unibyte buffer bug (bug#70007)

Fix json-insert unibyte buffer bug (bug#70007)

Previously, a unibyte target buffer could be put in an incorrect state
if json-insert was used to insert non-ASCII characters.

* src/json.c (Fjson_insert): Simplify.  Don't attempt to decode the data
being inserted: it is guaranteed to be correct UTF-8 and is correct for
both unibyte and multibyte buffers.
* test/src/json-tests.el (json-serialize/roundtrip)
(json-serialize/roundtrip-scalars): Extend tests.

(cherry picked from commit 617debf67392946b4b42fdf364c69da6f094a840)