fix: bridge codex image generation over responses

backend/internal/service/openai_gateway_service_test.go

@@ -18,6 +18,7 @@ import (
 	"github.com/cespare/xxhash/v2"
 	"github.com/gin-gonic/gin"
 	"github.com/stretchr/testify/require"
+	"github.com/tidwall/gjson"
 )
 
 // 编译期接口断言
@@ -1880,6 +1881,33 @@ func TestHandleSSEToJSON_CompletedEventReturnsJSON(t *testing.T) {
 	require.NotContains(t, rec.Body.String(), "data:")
 }
 
+func TestHandleSSEToJSON_ReconstructsImageGenerationOutputItemDone(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+	rec := httptest.NewRecorder()
+	c, _ := gin.CreateTestContext(rec)
+	c.Request = httptest.NewRequest(http.MethodPost, "/", nil)
+
+	svc := &OpenAIGatewayService{cfg: &config.Config{}}
+	resp := &http.Response{
+		StatusCode: http.StatusOK,
+		Header:     http.Header{"Content-Type": []string{"text/event-stream"}},
+	}
+	body := []byte(strings.Join([]string{
+		`data: {"type":"response.output_item.done","item":{"id":"ig_123","type":"image_generation_call","result":"aGVsbG8=","revised_prompt":"draw a cat","output_format":"png"}}`,
+		`data: {"type":"response.completed","response":{"id":"resp_img","model":"gpt-5.4","output":[],"usage":{"input_tokens":7,"output_tokens":9,"output_tokens_details":{"image_tokens":4}}}}`,
+		`data: [DONE]`,
+	}, "\n"))
+
+	usage, err := svc.handleSSEToJSON(resp, c, body, "gpt-5.4", "gpt-5.4")
+	require.NoError(t, err)
+	require.NotNil(t, usage)
+	require.Equal(t, 4, usage.ImageOutputTokens)
+	require.NotContains(t, rec.Body.String(), "data:")
+	require.Equal(t, "image_generation_call", gjson.Get(rec.Body.String(), "output.0.type").String())
+	require.Equal(t, "aGVsbG8=", gjson.Get(rec.Body.String(), "output.0.result").String())
+	require.Equal(t, "draw a cat", gjson.Get(rec.Body.String(), "output.0.revised_prompt").String())
+}
+
 func TestHandleSSEToJSON_NoFinalResponseKeepsSSEBody(t *testing.T) {
 	gin.SetMode(gin.TestMode)
 	rec := httptest.NewRecorder()