Ensure we validate context_mapping in condition.plugin

Identified in review for 📌 PersonalizationSegment config entity Active .

Although we're working on Personalization in parallel to trying to get a stable XB 1.0.0 released, the former is not a blocker to the latter. If Personalization itself isn't sufficiently stable by the time we're ready to release XB 1.0.0, we can put it behind a feature flag.