fix(openai): prefer concrete GPT-5.6 test model

backend/internal/handler/admin/account_handler_available_models_test.go

@@ -213,6 +213,38 @@ func TestAccountHandlerGetAvailableModels_OpenAIOAuthPassthroughFallsBackToDefau
 	require.NotEqual(t, "gpt-5", resp.Data[0].ID)
 }
 
+func TestAccountHandlerGetAvailableModels_OpenAIAPIKeyDefaultsToConcreteGPT56Sol(t *testing.T) {
+	svc := &availableModelsAdminService{
+		stubAdminService: newStubAdminService(),
+		account: service.Account{
+			ID:       46,
+			Name:     "openai-apikey",
+			Platform: service.PlatformOpenAI,
+			Type:     service.AccountTypeAPIKey,
+			Status:   service.StatusActive,
+			Credentials: map[string]any{
+				"api_key": "test-key",
+			},
+		},
+	}
+	router := setupAvailableModelsRouter(svc)
+
+	rec := httptest.NewRecorder()
+	req := httptest.NewRequest(http.MethodGet, "/api/v1/admin/accounts/46/models", nil)
+	router.ServeHTTP(rec, req)
+
+	require.Equal(t, http.StatusOK, rec.Code)
+
+	var resp struct {
+		Data []struct {
+			ID string `json:"id"`
+		} `json:"data"`
+	}
+	require.NoError(t, json.Unmarshal(rec.Body.Bytes(), &resp))
+	require.NotEmpty(t, resp.Data)
+	require.Equal(t, "gpt-5.6-sol", resp.Data[0].ID)
+}
+
 func TestAccountHandlerGetAvailableModels_OpenAISparkShadowReturnsMappingModels(t *testing.T) {
 	parentID := int64(100)
 	svc := &availableModelsAdminService{