fix(email-renderer): export builder rows as valid MJML columns

packages/email-renderer/src/blocks.ts

@@ -158,8 +158,13 @@ export const sectionBlock = defineBlock({
         `<table role="presentation" width="100%" cellpadding="0" cellspacing="0" border="0"><tr><td class="section"><table role="presentation" width="100%" cellpadding="0" cellspacing="0" border="0">${slots.content}</table></td></tr></table>`,
         node,
       ),
-    mjml: ({ node, slots }) =>
-      `<mj-section background-color="${html(attrs(node).background)}" padding="${html(attrs(node).padding)}"><mj-column>${slots.content}</mj-column></mj-section>`,
+    mjml: ({ node, slots }) => {
+      const children = node.slots.content ?? [];
+      const body = children.length === 1 && children[0]?.type === "layout.columns"
+        ? slots.content
+        : `<mj-column>${slots.content}</mj-column>`;
+      return `<mj-section background-color="${html(attrs(node).background)}" padding="${html(attrs(node).padding)}">${body}</mj-section>`;
+    },
   },
   textRenderer: ({ slots }) => `${slots.content}\n`,
 } satisfies BlockDefinition);