Output the final AST in explain query

Author: abyss7

ydb/core/kqp/host/kqp_explain_prepared.cpp

@@ -66,7 +66,7 @@ class TKqpExplainPreparedTransformer : public NYql::TGraphTransformerBase {
 
         PhyQuerySetTxPlans(query, TKqpPhysicalQuery(TransformCtx->ExplainTransformerInput), std::move(TxResults),
             ctx, Cluster, TransformCtx->Tables, TransformCtx->Config, TypeCtx, OptimizeCtx);
-        query.SetQueryAst(KqpExprToPrettyString(*TransformCtx->ExplainTransformerInput, ctx));
+        query.SetQueryAst(KqpExprToPrettyString(*input, ctx));
 
         TransformCtx->ExplainTransformerInput = nullptr;
         return TStatus::Ok;