fix: 恢复腾讯云账单同步并完善成本预测

- 接入腾讯云单日账单、全局限频和指数退避

- 修正月末费用预测并增加账期提示和费用归因

- 补充腾讯云限频及账单归因回归测试

backend/cloud/tencent_test.go

@@ -0,0 +1,28 @@
+package cloud
+
+import (
+	"errors"
+	"testing"
+	"time"
+)
+
+// TestTencentRetryDelay 验证限频错误指数退避与最大重试次数。 // Logan.Li
+func TestTencentRetryDelay(t *testing.T) {
+	rateLimitErr := &tencentAPIError{Code: "RequestLimitExceeded", Message: "frequency limit"}
+	delay, retry := tencentRetryDelay(rateLimitErr, 0)
+	if !retry || delay != 500*time.Millisecond {
+		t.Fatalf("首次限频退避错误: retry=%v delay=%s", retry, delay)
+	}
+
+	delay, retry = tencentRetryDelay(rateLimitErr, 2)
+	if !retry || delay != 2*time.Second {
+		t.Fatalf("第三次限频退避错误: retry=%v delay=%s", retry, delay)
+	}
+
+	if _, retry = tencentRetryDelay(rateLimitErr, tencentMaxAttempts-1); retry {
+		t.Fatal("达到最大次数后不应继续重试")
+	}
+	if _, retry = tencentRetryDelay(errors.New("鉴权失败"), 0); retry {
+		t.Fatal("非限频错误不应重试")
+	}
+}