Fix mock implementation to handle nil error values

- Updated MockProtocolProvider.GetCurrentBlockNumber to safely handle nil error returns
- Fixed interface conversion panic when mock returns nil for error parameter
- This resolves the test failure: 'interface conversion: interface is nil, not error'

Author: GheisMohammadi

api/service/synchronize/stagedstreamsync/block_number_cache_test.go

@@ -20,7 +20,7 @@ type MockProtocolProvider struct {
 
 func (m *MockProtocolProvider) GetCurrentBlockNumber(ctx context.Context, opts ...syncproto.Option) (uint64, sttypes.StreamID, error) {
 	args := m.Called(ctx, opts)
-	return args.Get(0).(uint64), args.Get(1).(sttypes.StreamID), args.Get(2).(error)
+	return args.Get(0).(uint64), args.Get(1).(sttypes.StreamID), func() error { if err := args.Get(2); err != nil { return err.(error) }; return nil }()
 }
 
 func TestNewBlockNumberCache(t *testing.T) {