fix(buyer-tasks): add emailError to route test fixture

The BuyerTaskGroup type gained an emailError field in the previous
commit. This pre-existing fixture constructs a BuyerTaskGroup
literal and now needs the field to compile. No behaviour change
in the test — emailError: null matches a clean draft.

Co-Authored-By: Claude <noreply@anthropic.com>

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

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

@@ -34,6 +34,7 @@ describe("GET /api/v1/buyer-tasks", () => {
         poStatus: null,
         poNumber: null,
         supplierEmail: null,
+        emailError: null,
         step: "needs_po",
       },
     ]);