test: 为代码审核修复添加详细单元测试（7个测试文件，50+测试用例）

新增测试文件:
- cors_test.go: CORS 条件化头部测试（12个测试，覆盖白名单/黑名单/通配符/凭证/多源/Vary）
- gateway_helper_backoff_test.go: nextBackoff 退避测试（6个测试+基准，验证指数增长/边界/抖动/收敛）
- billing_cache_jitter_test.go: jitteredTTL 抖动测试（5个测试+基准，验证范围/上界/方差/均值）
- subscription_calculate_progress_test.go: calculateProgress 纯函数测试（9个测试，覆盖日/周/月限额/超限截断/过期）
- openai_gateway_handler_test.go: SSE JSON 转义测试（7个子用例，验证双引号/反斜杠/换行符安全）

更新测试文件:
- response_transformer_test.go: 增强 generateRandomID 测试（7个测试，含并发/字符集/降级计数器）
- security_headers_test.go: 适配 GenerateNonce 新签名
- api_key_auth_test.go: 适配 NewSubscriptionService 新参数

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/internal/handler/openai_gateway_handler_test.go

@@ -0,0 +1,104 @@
+package handler
+
+import (
+	"encoding/json"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+
+	"github.com/gin-gonic/gin"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestOpenAIHandleStreamingAwareError_JSONEscaping(t *testing.T) {
+	tests := []struct {
+		name    string
+		errType string
+		message string
+	}{
+		{
+			name:    "包含双引号的消息",
+			errType: "server_error",
+			message: `upstream returned "invalid" response`,
+		},
+		{
+			name:    "包含反斜杠的消息",
+			errType: "server_error",
+			message: `path C:\Users\test\file.txt not found`,
+		},
+		{
+			name:    "包含双引号和反斜杠的消息",
+			errType: "upstream_error",
+			message: `error parsing "key\value": unexpected token`,
+		},
+		{
+			name:    "包含换行符的消息",
+			errType: "server_error",
+			message: "line1\nline2\ttab",
+		},
+		{
+			name:    "普通消息",
+			errType: "upstream_error",
+			message: "Upstream service temporarily unavailable",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			gin.SetMode(gin.TestMode)
+			w := httptest.NewRecorder()
+			c, _ := gin.CreateTestContext(w)
+			c.Request = httptest.NewRequest(http.MethodGet, "/", nil)
+
+			h := &OpenAIGatewayHandler{}
+			h.handleStreamingAwareError(c, http.StatusBadGateway, tt.errType, tt.message, true)
+
+			body := w.Body.String()
+
+			// 验证 SSE 格式：event: error\ndata: {JSON}\n\n
+			assert.True(t, strings.HasPrefix(body, "event: error\n"), "应以 'event: error\\n' 开头")
+			assert.True(t, strings.HasSuffix(body, "\n\n"), "应以 '\\n\\n' 结尾")
+
+			// 提取 data 部分
+			lines := strings.Split(strings.TrimSuffix(body, "\n\n"), "\n")
+			require.Len(t, lines, 2, "应有 event 行和 data 行")
+			dataLine := lines[1]
+			require.True(t, strings.HasPrefix(dataLine, "data: "), "第二行应以 'data: ' 开头")
+			jsonStr := strings.TrimPrefix(dataLine, "data: ")
+
+			// 验证 JSON 合法性
+			var parsed map[string]any
+			err := json.Unmarshal([]byte(jsonStr), &parsed)
+			require.NoError(t, err, "JSON 应能被成功解析，原始 JSON: %s", jsonStr)
+
+			// 验证结构
+			errorObj, ok := parsed["error"].(map[string]any)
+			require.True(t, ok, "应包含 error 对象")
+			assert.Equal(t, tt.errType, errorObj["type"])
+			assert.Equal(t, tt.message, errorObj["message"])
+		})
+	}
+}
+
+func TestOpenAIHandleStreamingAwareError_NonStreaming(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+	w := httptest.NewRecorder()
+	c, _ := gin.CreateTestContext(w)
+	c.Request = httptest.NewRequest(http.MethodGet, "/", nil)
+
+	h := &OpenAIGatewayHandler{}
+	h.handleStreamingAwareError(c, http.StatusBadGateway, "upstream_error", "test error", false)
+
+	// 非流式应返回 JSON 响应
+	assert.Equal(t, http.StatusBadGateway, w.Code)
+
+	var parsed map[string]any
+	err := json.Unmarshal(w.Body.Bytes(), &parsed)
+	require.NoError(t, err)
+	errorObj, ok := parsed["error"].(map[string]any)
+	require.True(t, ok)
+	assert.Equal(t, "upstream_error", errorObj["type"])
+	assert.Equal(t, "test error", errorObj["message"])
+}