Fix FORMAT clause handling in EXPLAIN statement output

When EXPLAIN wraps a SELECT query with FORMAT clause, the FORMAT
identifier should be a child of the Explain node, not the inner
SelectWithUnionQuery. Updated explainExplainQuery to extract the
format from the inner SelectQuery and output it as a sibling.

Fixes 17 statements across 13 tests.

Author: claude

internal/explain/statements.go

@@ -613,11 +613,30 @@ func explainExplainQuery(sb *strings.Builder, n *ast.ExplainQuery, indent string
 		}
 		return
 	}
-	// Count children: settings (if present) + statement
+
+	// Check if inner statement has FORMAT clause - this should be output as child of Explain
+	var format *ast.Identifier
+	if swu, ok := n.Statement.(*ast.SelectWithUnionQuery); ok {
+		for _, sel := range swu.Selects {
+			if sq, ok := sel.(*ast.SelectQuery); ok && sq.Format != nil {
+				format = sq.Format
+				// Temporarily nil out the format so it's not output by SelectWithUnionQuery
+				sq.Format = nil
+				defer func() { sq.Format = format }()
+				break
+			}
+		}
+	}
+
+	// Count children: settings (if present) + statement + format (if present)
 	children := 1
 	if n.HasSettings {
 		children++
 	}
+	if format != nil {
+		children++
+	}
+
 	// At top level (depth 0), ClickHouse outputs "Explain EXPLAIN <TYPE>"
 	// Nested in subqueries, it outputs "Explain <TYPE>"
 	if depth == 0 {
@@ -629,6 +648,9 @@ func explainExplainQuery(sb *strings.Builder, n *ast.ExplainQuery, indent string
 		fmt.Fprintf(sb, "%s Set\n", indent)
 	}
 	Node(sb, n.Statement, depth+1)
+	if format != nil {
+		fmt.Fprintf(sb, "%s Identifier %s\n", indent, format.Parts[len(format.Parts)-1])
+	}
 }
 
 func explainShowQuery(sb *strings.Builder, n *ast.ShowQuery, indent string) {

parser/testdata/01470_explain/metadata.json

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

parser/testdata/02271_fix_column_matcher_and_column_transformer/metadata.json

@@ -1,5 +1 @@
-{
-  "explain_todo": {
-    "stmt7": true
-  }
-}
+{}

parser/testdata/02421_exponential_join_rewrite_21557/metadata.json

@@ -1,5 +1 @@
-{
-  "explain_todo": {
-    "stmt27": true
-  }
-}
+{}

parser/testdata/02678_explain_pipeline_graph_with_projection/metadata.json

@@ -1,8 +1 @@
-{
-  "explain_todo": {
-    "stmt4": true,
-    "stmt5": true,
-    "stmt6": true,
-    "stmt7": true
-  }
-}
+{}

parser/testdata/02704_storage_merge_explain_graph_crash/metadata.json

@@ -1,6 +1,5 @@
 {
   "explain_todo": {
-    "stmt11": true,
     "stmt12": true
   }
 }

parser/testdata/02707_skip_index_with_in/metadata.json

@@ -1 +1 @@
-{"explain_todo":{"stmt4":true}}
+{}

parser/testdata/02780_final_streams_data_skipping_index/metadata.json

@@ -1,6 +1 @@
-{
-  "explain_todo": {
-    "stmt7": true,
-    "stmt8": true
-  }
-}
+{}

parser/testdata/02788_fix_logical_error_in_sorting/metadata.json

@@ -1,5 +1 @@
-{
-  "explain_todo": {
-    "stmt12": true
-  }
-}
+{}

parser/testdata/03023_group_by_use_nulls_analyzer_crashes/metadata.json

@@ -1,5 +1 @@
-{
-  "explain_todo": {
-    "stmt11": true
-  }
-}
+{}