fix(billing): gofmt channel.go and drop the redundant response-model hint

- channel.go: 常量块里插入注释后 gofmt 会把 BillingModelSourceResponse 单独成组，
  原写法沿用了上一组的对齐空格，CI 的 gofmt 检查因此失败
  (internal/service/channel.go:45: File is not properly formatted)。
- 撤掉渠道表单里新加的那条提示：说明本就多余，且"只降不升"只在"相对基线收费"
  这个口径下成立，容易被读成"上游返回更贵的模型也不会多收"，反而误导。

frontend/src/views/admin/ChannelsView.vue

@@ -226,9 +226,6 @@
               <p class="mt-1 text-xs text-gray-400">
                 {{ t('admin.channels.form.billingModelSourceHint', 'Controls which model name is used for pricing lookup') }}
               </p>
-              <p v-if="form.billing_model_source === 'response_model'" class="mt-1 text-xs text-amber-500">
-                {{ t('admin.channels.form.billingModelSourceResponseWarning', 'Pricing follows the model declared by the upstream response. It can only lower the charge, never raise it — only enable this for upstreams you trust.') }}
-              </p>
             </div>
 
             <!-- Platform Management -->