[KQP Constant folding] Add folding for expressions inside AsStruct (#19777)

Author: denis0x0D

ydb/core/kqp/opt/kqp_constant_folding_transformer.cpp

@@ -39,6 +39,13 @@ namespace {
             return;
         }
 
+        if (TCoAsStruct::Match(input.Get())) {
+            for (auto child : TExprBase(input).Cast<TCoAsStruct>()) {
+                ExtractConstantExprs(child.Item(1).Ptr(), replaces, ctx);
+            }
+            return;
+        }
+
         if (input->IsCallable() && input->Content() != "EvaluateExpr") {
             if (input->ChildrenSize() >= 1) {
                 for (size_t i = 0; i < input->ChildrenSize(); i++) {

ydb/core/kqp/ut/olap/kqp_olap_ut.cpp

@@ -3889,7 +3889,6 @@ Y_UNIT_TEST_SUITE(KqpOlap) {
     }
 
     Y_UNIT_TEST(GroupByWithMakeDatetime) {
-        return; // TODO: fix me
         auto settings = TKikimrSettings()
             .SetWithSampleTables(false);
         TKikimrRunner kikimr(settings);