fix(openai): 统一专属倍率计费链路并补齐回归测试

抽取共享的用户分组专属倍率解析器，统一缓存、singleflight 与回退逻辑。\n\n让 OpenAI 独立计费链路复用专属倍率解析，修复 usage 记录与实际扣费未命中用户专属倍率的问题。\n\n补齐 OpenAI 计费与解析器单元测试，并修复全量回归中暴露的 lint 阻塞项。\n\nCo-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/internal/service/openai_gateway_service_test.go

@@ -1576,3 +1576,27 @@ func TestHandleOAuthSSEToJSON_NoFinalResponseKeepsSSEBody(t *testing.T) {
 	require.Contains(t, rec.Header().Get("Content-Type"), "text/event-stream")
 	require.Contains(t, rec.Body.String(), `data: {"type":"response.in_progress"`)
 }
+
+func TestHandleOAuthSSEToJSON_ResponseFailedReturnsProtocolError(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.failed","error":{"message":"upstream rejected request"}}`,
+		`data: [DONE]`,
+	}, "\n"))
+
+	usage, err := svc.handleOAuthSSEToJSON(resp, c, body, "gpt-4o", "gpt-4o")
+	require.Nil(t, usage)
+	require.Error(t, err)
+	require.Equal(t, http.StatusBadGateway, rec.Code)
+	require.Contains(t, rec.Body.String(), "upstream rejected request")
+	require.Contains(t, rec.Header().Get("Content-Type"), "application/json")
+}