test(server): add undeliverable:[] to processRelayEvents mock (#18)

Follow-up from Task 2 code review. Keeps the test mock aligned with
the widened return type even though vi.mock doesn't structurally
typecheck the factory.

packages/server/src/utils/federationPeerActivation.test.ts

@@ -32,7 +32,7 @@ vi.mock('../utils/federationAuth.js', () => ({
 }));
 
 vi.mock('../routes/federation.js', () => ({
-  processRelayEvents: vi.fn().mockResolvedValue({ accepted: [], rejected: [] }),
+  processRelayEvents: vi.fn().mockResolvedValue({ accepted: [], rejected: [], undeliverable: [] }),
 }));
 
 vi.mock('../ws/handler.js', () => ({