fix(ops): 修复日志级别过滤并增强OpenAI错误诊断日志

- 移除 warn 级别下 access info 的强制入库补写，确保运行时日志级别真实生效

- 将 OpenAI fallback matched 与 passthrough 断流提示按需求降级为 info

- 为 codex_cli_only 与 instructions required 场景补充请求诊断字段（含 User-Agent）

- 出于安全考虑移除请求体预览，仅保留 request_body_size 与白名单头信息

- 新增/更新回归测试，覆盖 Forward 入口到日志落库链路

backend/internal/service/pricing_service_test.go

@@ -33,3 +33,21 @@ func TestGetModelPricing_Gpt53CodexFallbackStillUsesGpt52Codex(t *testing.T) {
 	got := svc.GetModelPricing("gpt-5.3-codex")
 	require.Same(t, gpt52CodexPricing, got)
 }
+
+func TestGetModelPricing_OpenAIFallbackMatchedLoggedAsInfo(t *testing.T) {
+	logSink, restore := captureStructuredLog(t)
+	defer restore()
+
+	gpt52CodexPricing := &LiteLLMModelPricing{InputCostPerToken: 2}
+	svc := &PricingService{
+		pricingData: map[string]*LiteLLMModelPricing{
+			"gpt-5.2-codex": gpt52CodexPricing,
+		},
+	}
+
+	got := svc.GetModelPricing("gpt-5.3-codex")
+	require.Same(t, gpt52CodexPricing, got)
+
+	require.True(t, logSink.ContainsMessageAtLevel("[Pricing] OpenAI fallback matched gpt-5.3-codex -> gpt-5.2-codex", "info"))
+	require.False(t, logSink.ContainsMessageAtLevel("[Pricing] OpenAI fallback matched gpt-5.3-codex -> gpt-5.2-codex", "warn"))
+}