[client] Reinject captured first packet on lazy connection activation (#6572)

client/internal/engine_test.go

@@ -178,6 +178,10 @@ func (m *MockWGIface) LastActivities() map[string]monotime.Time {
 	return nil
 }
 
+func (m *MockWGIface) MTU() uint16 {
+	return 1280
+}
+
 func (m *MockWGIface) SetPresharedKey(peerKey string, psk wgtypes.Key, updateOnly bool) error {
 	return nil
 }