Extensions aren’t allowed in $unionWith until featureFlagExtensionViewsAndUnionWith is enabled. In DocumentSourceExtensionOptimizable::create():

static boost::intrusive_ptr<DocumentSourceExtensionOptimizable> create(...) {
    if (expCtx->getInUnionWith() &&
        !feature_flags::gFeatureFlagExtensionViewsAndUnionWith.isEnabled()) {
           throwIfrKickbackIfNecessary(name, isExtensionSearchStage, gFeatureFlagSearchExtension,search_metrics::inUnionWithKickbackRetries, "...");
    }
    //...
}

Integration Tests:

• $search/$searchMeta inside $unionWith with the appropriate flag disabled.
  • IFR kickback fires and the query retries.
  • The appropriate metric is incremented.
• $search/$searchMeta inside $unionWith with the appropriate flag enabled.
  • No IFR kickback or metric increment.