The field-type union on EntitySchema/FieldSchema.type omitted 'object' and
'datetime', so codegen-emitted seed metadata for JSON/Struct (object) and
datetime entity fields failed to typecheck (TS2322). Add both members and
extend the value-generator switch (datetime -> date value, object -> {}).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>