fix(openai): tighten responses stream account tests

backend/internal/service/account_test_service_openai_test.go

@@ -125,6 +125,31 @@ func TestAccountTestService_OpenAISuccessPersistsSnapshotFromHeaders(t *testing.
 	require.Contains(t, recorder.Body.String(), "test_complete")
 }
 
+func TestAccountTestService_OpenAIStreamEOFBeforeCompletedFails(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+	ctx, recorder := newTestContext()
+
+	resp := newJSONResponse(http.StatusOK, "")
+	resp.Body = io.NopCloser(strings.NewReader(`data: {"type":"response.output_text.delta","delta":"hi"}
+
+`))
+
+	upstream := &queuedHTTPUpstream{responses: []*http.Response{resp}}
+	svc := &AccountTestService{httpUpstream: upstream}
+	account := &Account{
+		ID:          90,
+		Platform:    PlatformOpenAI,
+		Type:        AccountTypeOAuth,
+		Concurrency: 1,
+		Credentials: map[string]any{"access_token": "test-token"},
+	}
+
+	err := svc.testOpenAIAccountConnection(ctx, account, "gpt-5.4", "", "")
+	require.Error(t, err)
+	require.Contains(t, recorder.Body.String(), "response.completed")
+	require.NotContains(t, recorder.Body.String(), `"success":true`)
+}
+
 func TestAccountTestService_OpenAI429PersistsSnapshotAndRateLimitState(t *testing.T) {
 	gin.SetMode(gin.TestMode)
 	ctx, _ := newTestContext()