materialize-webhook: require collection keys in field selection

Previously, collection keys (and all other fields) were forbidden from
field selection. Since the runtime uses field selection to determine the
composite keys that are sent into Load & are reduced then sent as Store
requests, only a small subset of the full collection was being sent to
the connector. This resulted in data loss.

Adding this case statement helps prevent this unintentional document
reduction from happening.

materialize-webhook/driver.go

@@ -126,6 +126,9 @@ func (driver) Validate(ctx context.Context, req *pm.Request_Validate) (*pm.Respo
 			case projection.IsRootDocumentProjection():
 				constraint.Type = pm.Response_Validated_Constraint_LOCATION_REQUIRED
 				constraint.Reason = "The root document must be materialized"
+			case projection.IsPrimaryKey:
+				constraint.Type = pm.Response_Validated_Constraint_LOCATION_REQUIRED
+				constraint.Reason = "Document keys must be included"
 			default:
 				constraint.Type = pm.Response_Validated_Constraint_FIELD_FORBIDDEN
 				constraint.Reason = "Webhooks only materialize the full document"