Fix SHOW DATABASES EXPLAIN AST output to match ClickHouse

ClickHouse maps SHOW DATABASES to ShowTables in EXPLAIN AST output,
similar to SHOW SETTINGS. Update explainShowQuery to handle this case.

Enables test: 02902_show_databases_limit

Author: claude

internal/explain/statements.go

@@ -318,8 +318,8 @@ func explainExplainQuery(sb *strings.Builder, n *ast.ExplainQuery, indent string
 func explainShowQuery(sb *strings.Builder, n *ast.ShowQuery, indent string) {
 	// ClickHouse maps certain SHOW types to ShowTables in EXPLAIN AST
 	showType := strings.Title(strings.ToLower(string(n.ShowType)))
-	// SHOW SETTINGS is displayed as ShowTables in ClickHouse
-	if showType == "Settings" {
+	// SHOW SETTINGS and SHOW DATABASES are displayed as ShowTables in ClickHouse
+	if showType == "Settings" || showType == "Databases" {
 		showType = "Tables"
 	}
 	fmt.Fprintf(sb, "%sShow%s\n", indent, showType)

parser/testdata/02902_show_databases_limit/metadata.json

@@ -1 +1 @@
-{"todo": true}
+{}