1.9.3 broke snapshots with compound unique keys

[agrueneberg]

The new snapshot code introduced in #509 does not account for compound unique keys, introducing string arrays (e.g., ['date', 'team_id', 'person_id']) as dbt_unique_key in both snapshotted_data and insertions_source_data. When combined with the LEFT JOIN used in insertions, this leads to an unbounded Cartesian product — resulting in infinite insertions until disk space is exhausted.