test: assert attachments field in heartbeat-context response

Add missing assertion for the empty attachments array in the
heartbeat-context test to verify the field mapping is present.

server/src/__tests__/issues-goal-context-routes.test.ts

@@ -199,5 +199,6 @@ describe("issue goal context routes", () => {
       }),
     );
     expect(mockGoalService.getDefaultCompanyGoal).not.toHaveBeenCalled();
+    expect(res.body.attachments).toEqual([]);
   });
 });