loadGraph() trusted the persisted memory file and pushed entities and
relations without validating their fields. A corrupted or legacy entry
(e.g. an entity missing entityType, or an observation that is not a string)
would reach searchNodes and crash with "Cannot read properties of undefined
(reading 'toLowerCase')".
Validate each line against the existing EntitySchema/RelationSchema and skip
malformed entries with a warning, so the in-memory graph only ever contains
well-formed data. Malformed JSON lines are skipped as well.
Fixes#2044
c3d8e43d1f