Fix WITH FILL STEP output format in ORDER BY

When ORDER BY has WITH fill step N (without FROM/TO), ClickHouse
outputs the step value directly as a Literal child of OrderByElement,
not wrapped in a FillModifier node.

Previously we used FillModifier for step-only cases, but ClickHouse
only uses FillModifier when FROM/TO contain complex expressions.

Fixes 19+ statements across 6 tests related to WITH FILL.

Author: claude

internal/explain/select.go

@@ -194,10 +194,9 @@ func explainOrderByElement(sb *strings.Builder, n *ast.OrderByElement, indent st
 	hasFromOrTo := n.FillFrom != nil || n.FillTo != nil
 	hasComplexFillExpr := hasFromOrTo && (isComplexExpr(n.FillFrom) || isComplexExpr(n.FillTo))
 
-	// Use FillModifier when:
-	// 1. Only STEP is present (no FROM/TO), or
-	// 2. FROM/TO contain complex expressions (not simple literals)
-	useFillModifier := n.WithFill && ((n.FillStep != nil && !hasFromOrTo) || hasComplexFillExpr)
+	// Use FillModifier when FROM/TO contain complex expressions (not simple literals)
+	// When only STEP is present, output it directly as a child (no FillModifier)
+	useFillModifier := n.WithFill && hasComplexFillExpr
 
 	if useFillModifier {
 		// Use FillModifier wrapper

parser/testdata/00995_order_by_with_fill/metadata.json

@@ -1,6 +1,5 @@
 {
   "explain_todo": {
-    "stmt16": true,
     "stmt19": true,
     "stmt6": true,
     "stmt8": true,

parser/testdata/01034_with_fill_and_push_down_predicate/metadata.json

@@ -1,5 +1 @@
-{
-  "explain_todo": {
-    "stmt1": true
-  }
-}
+{}

parser/testdata/01868_order_by_fill_with_datetime64/metadata.json

@@ -1,6 +1 @@
-{
-  "explain_todo": {
-    "stmt1": true,
-    "stmt2": true
-  }
-}
+{}

parser/testdata/02112_with_fill_interval/metadata.json

@@ -1 +1,8 @@
-{"explain_todo":{"stmt10":true,"stmt12":true,"stmt14":true,"stmt15":true,"stmt17":true,"stmt19":true,"stmt21":true,"stmt23":true,"stmt24":true,"stmt31":true,"stmt33":true,"stmt35":true,"stmt37":true,"stmt39":true,"stmt41":true,"stmt43":true,"stmt50":true,"stmt52":true,"stmt8":true}}
+{
+  "explain_todo": {
+    "stmt14": true,
+    "stmt23": true,
+    "stmt50": true,
+    "stmt52": true
+  }
+}

parser/testdata/02730_with_fill_by_sorting_prefix/metadata.json

@@ -1,7 +1,5 @@
 {
   "explain_todo": {
-    "stmt13": true,
-    "stmt19": true,
     "stmt20": true,
     "stmt21": true,
     "stmt22": true,
@@ -14,7 +12,6 @@
     "stmt29": true,
     "stmt30": true,
     "stmt31": true,
-    "stmt32": true,
-    "stmt6": true
+    "stmt32": true
   }
 }

parser/testdata/03366_with_fill_dag/metadata.json

@@ -1,6 +1,5 @@
 {
   "explain_todo": {
-    "stmt1": true,
     "stmt2": true
   }
 }