fix: mock /healthz in runtime service e2e test

TestServiceStartGameEndToEnd's httptest server had no handler for
/healthz, the path engineclient.Healthz probes after a runtime
container starts. Without it the runtime never transitions out of
starting state and the test fails on its 5s deadline. Surfaced by
introducing CI that runs the backend service tests outside the
integration harness.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/internal/runtime/service_e2e_test.go

@@ -200,6 +200,8 @@ func TestServiceStartGameEndToEnd(t *testing.T) {
 	engineSrv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		w.Header().Set("Content-Type", "application/json")
 		switch r.URL.Path {
+		case "/healthz":
+			w.WriteHeader(http.StatusOK)
 		case "/api/v1/admin/init":
 			_ = json.NewEncoder(w).Encode(rest.StateResponse{ID: gameID, Turn: 0, Players: []rest.PlayerState{{RaceName: "Alpha", Planets: 3, Population: 10}}})
 		case "/api/v1/admin/status":