Fix Walk logic for DistinctOn clause (#195)

Add support for walking DistinctOn AST nodes in the Walk function to ensure proper traversal of SELECT DISTINCT ON expressions.

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Co-authored-by: Claude <[email protected]>

Author: git-hulk

parser/walk.go

@@ -22,6 +22,9 @@ func Walk(node Expr, fn WalkFunc) bool {
 		if !Walk(n.With, fn) {
 			return false
 		}
+		if !Walk(n.DistinctOn, fn) {
+			return false
+		}
 		if !Walk(n.Top, fn) {
 			return false
 		}
@@ -1421,6 +1424,12 @@ func Walk(node Expr, fn WalkFunc) bool {
 		if !Walk(n.Target, fn) {
 			return false
 		}
+	case *DistinctOn:
+		for _, ident := range n.Idents {
+			if !Walk(ident, fn) {
+				return false
+			}
+		}
 	}
 	return true
 }