Fix EXPLAIN PLAN type output in explain format

EXPLAIN PLAN is the default type and shouldn't be shown in the explain
output. Now only non-default types (AST, SYNTAX, etc.) are shown.

This fixes 20+ statements across multiple tests including:
- 03652_explain_input_header
- 03591_optimize_prewhere_row_policy
- 03405_merge_filter_into_join
- 03721_aggregate_projection_actions_dag

Author: claude

internal/explain/statements.go

@@ -648,9 +648,9 @@ func explainSystemQuery(sb *strings.Builder, n *ast.SystemQuery, indent string)
 }
 
 func explainExplainQuery(sb *strings.Builder, n *ast.ExplainQuery, indent string, depth int) {
-	// Determine the type string - only show if explicitly specified
+	// Determine the type string - only show if explicitly specified AND not PLAN (default)
 	typeStr := ""
-	if n.ExplicitType {
+	if n.ExplicitType && n.ExplainType != ast.ExplainPlan {
 		typeStr = " " + string(n.ExplainType)
 	}
 

parser/testdata/02494_query_cache_explain/metadata.json

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

parser/testdata/02514_analyzer_drop_join_on/metadata.json

@@ -1,7 +1 @@
-{
-  "explain_todo": {
-    "stmt18": true,
-    "stmt19": true,
-    "stmt20": true
-  }
-}
+{}

parser/testdata/03282_memory_transaction_crash/metadata.json

@@ -1,5 +1 @@
-{
-  "explain_todo": {
-    "stmt4": true
-  }
-}
+{}

parser/testdata/03405_merge_filter_into_join/metadata.json

@@ -1,9 +1 @@
-{
-  "explain_todo": {
-    "stmt11": true,
-    "stmt13": true,
-    "stmt14": true,
-    "stmt15": true,
-    "stmt9": true
-  }
-}
+{}

parser/testdata/03591_optimize_prewhere_row_policy/metadata.json

@@ -1,8 +1 @@
-{
-  "explain_todo": {
-    "stmt12": true,
-    "stmt13": true,
-    "stmt15": true,
-    "stmt16": true
-  }
-}
+{}

parser/testdata/03652_explain_input_header/metadata.json

@@ -1,14 +1,6 @@
 {
   "explain_todo": {
-    "stmt10": true,
-    "stmt11": true,
-    "stmt12": true,
     "stmt13": true,
-    "stmt14": true,
-    "stmt15": true,
-    "stmt16": true,
-    "stmt17": true,
-    "stmt18": true,
-    "stmt9": true
+    "stmt18": true
   }
 }

parser/testdata/03721_aggregate_projection_actions_dag/metadata.json

@@ -1,5 +1 @@
-{
-  "explain_todo": {
-    "stmt5": true
-  }
-}
+{}