_parse_msg_content deserializes stored multimodal content (image/audio
blocks) back into a list. It treated ANY string starting with '[{' and
containing the substring "type" as serialized content, requiring only
that each element be a dict — never that "type" be a real content-block
kind. So a plain text message whose content happens to be a JSON array
of typed objects (e.g. a user pasting an API schema sample like
[{"type": "object", ...}]) was silently parsed from str into a list on
the next hydration, destroying the original string. This runs on every
session load from the DB (_db_to_session -> get_session). Restrict the
round-trip to non-empty lists whose every element is a dict whose
"type" is a recognized block kind (text/image/image_url/audio/...);
real multimodal content (verified: document_processor emits exactly
these) still round-trips, JSON-looking text is left untouched.
