fix: address audit execution failures and unsafe type conversions

pkg/writers/pgsql/templates.go

@@ -99,15 +99,15 @@ type AddColumnData struct {
 
 // AlterColumnTypeWithCheckData contains data for the guarded alter column
 // type template, which only alters existing columns whose live type
-// differs from the desired one, and falls back to renaming the old column
-// and adding a fresh one when the in-place conversion is not possible.
+// differs from the desired one. Conversion errors propagate unchanged.
 type AlterColumnTypeWithCheckData struct {
-	SchemaName      string
-	TableName       string
-	ColumnName      string
-	NewType         string
-	EquivalentTypes string
-	UsingExpr       string
+	SchemaName         string
+	TableName          string
+	ColumnName         string
+	NewType            string
+	EquivalentTypes    string
+	UsingExpr          string
+	PreflightPredicate string
 }
 
 // AlterColumnDefaultWithCheckData contains data for the guarded alter
@@ -332,7 +332,10 @@ func (te *TemplateExecutor) ExecuteAlterColumnTypeWithCheck(data AlterColumnType
 	if err != nil {
 		return "", fmt.Errorf("failed to execute alter_column_type_with_check template: %w", err)
 	}
-	return buf.String(), nil
+	body := strings.TrimSpace(buf.String())
+	body = strings.TrimPrefix(body, "DO $$")
+	body = strings.TrimSuffix(body, "$$;")
+	return dollarBlock(strings.TrimSpace(body), "relspec_alter_type"), nil
 }
 
 // ExecuteAlterColumnDefaultWithCheck executes the guarded alter column