Revert agent generations that cut off the control plane

internal/agent/agent_test.go

@@ -132,10 +132,10 @@ type fakeApplier struct {
 	lastGen int
 }
 
-func (f *fakeApplier) Apply(_ context.Context, cfg *config.Config) error {
+func (f *fakeApplier) Apply(_ context.Context, cfg *config.Config) (func() error, error) {
 	atomic.AddInt32(&f.count, 1)
 	f.lastGen = len(cfg.Rules)
-	return nil
+	return nil, nil
 }
 
 const renderedYAML = `generation: 7