fix(provider): scan rejects non-dict sort with InvalidArguments, never AttributeError
heid-bug-hunt panel (Gróa + Hulda, confirmed-from-code) caught that a truthy
non-dict `sort` (e.g. sort="updated_at" or sort=["updated_at"]) reached
`(sort or {}).get(...)` and crashed with AttributeError instead of the
InvalidArguments PRE-003 promises for malformed caller-controlled input. Add an
isinstance guard before field extraction. Test covers str/list/int sort values.
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