test(buyer-tasks): lock wire shape of mail_failed group in po_draft

The route is a pass-through, but the smoke test guards against a
future refactor silently dropping the new emailError field — the
operator's retry affordance depends on it.

Co-authored-by: Orca <help@stably.ai>

src/app/api/v1/buyer-tasks/route.test.ts

@@ -46,6 +46,36 @@ describe("GET /api/v1/buyer-tasks", () => {
     expect(body.groups[0].supplierName).toBe("Steel Co");
   });
 
+  it("returns mail_failed groups when tab=po_draft (operator can retry from buyer-tasks)", async () => {
+    vi.mocked(listBuyerTasks).mockResolvedValue([
+      {
+        groupId: "order-1::sup-1",
+        orderId: "order-1",
+        orderNumber: "ORD-001",
+        supplierId: "sup-1",
+        supplierName: "Steel Co",
+        customerName: "J. Smith",
+        companyName: "Acme",
+        items: [],
+        itemCount: 1,
+        totalCost: 50,
+        purchaseOrderId: "po-1",
+        poStatus: "mail_failed",
+        poNumber: "PO-26080301",
+        supplierEmail: "orders@steelco.example",
+        emailError: "[webhook] Webhook returned HTTP 502",
+        step: "po_draft",
+      },
+    ]);
+
+    const res = await GET(makeRequest("po_draft") as never);
+    expect(res.status).toBe(200);
+    const body = await res.json();
+    expect(body.groups).toHaveLength(1);
+    expect(body.groups[0].poStatus).toBe("mail_failed");
+    expect(body.groups[0].emailError).toMatch(/webhook/i);
+  });
+
   it("returns 400 on invalid tab", async () => {
     const res = await GET(makeRequest("invalid") as never);
     expect(res.status).toBe(400);