fix(email): validate MJML before exposing exports

packages/email-renderer/test/root-mjml.test.ts

@@ -5,6 +5,17 @@ import { renderEmail, validateMjml } from "../src/index.ts";
 const text = (id: string): BlockNode => ({ id, type: "content.text", version: 1, props: { text: id }, styles: {}, slots: {} });
 const columns = (id: string): BlockNode => ({ id, type: "layout.columns", version: 1, props: {}, styles: {}, slots: { columns: [0, 1].map(index => ({ id: `${id}-${index}`, type: "layout.column", version: 1, props: { width: "50%" }, styles: {}, slots: { content: [text(`${id}-text-${index}`)] } })) } });
 
+test("invalid nested MJML is diagnosed and omitted while HTML and text remain usable", async () => {
+  const document: BlockDocument = { schemaVersion: 1, kind: "email", meta: { subject: "Nested", lang: "en" }, theme: { tokens: {} }, root: { id: "root", type: "layout.email-document", version: 1, props: {}, styles: {}, slots: { content: [{ id: "outer", type: "layout.section", version: 1, props: {}, styles: {}, slots: { content: [{ id: "inner", type: "layout.section", version: 1, props: {}, styles: {}, slots: { content: [text("Retained nested content")] } }] } }] } } };
+  const rendered = await renderEmail(document, { mjml: true });
+  expect(rendered.ok).toBe(true);
+  if (!rendered.ok) return;
+  expect(rendered.value.exports?.mjml).toBeUndefined();
+  expect(rendered.value.warnings.some(warning => warning.code === "mjml.validation")).toBe(true);
+  expect(rendered.value.parts.html.content).toContain("Retained nested content");
+  expect(rendered.value.parts.text.content).toContain("Retained nested content");
+});
+
 test("root content and separate column rows produce valid ordered MJML without changing the document", async () => {
   const document: BlockDocument = { schemaVersion: 1, kind: "email", meta: { subject: "Root blocks", lang: "en" }, theme: { tokens: {} }, root: { id: "root", type: "layout.email-document", version: 1, props: {}, styles: {}, slots: { content: [text("intro"), columns("first"), columns("second"), { id: "cta", type: "content.button", version: 1, props: { label: "Act", href: "https://example.com" }, styles: {}, slots: {} }, text("end")] } } };
   const before = structuredClone(document);