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。

backend/internal/service/channel.go

@@ -42,7 +42,7 @@ const (
 	// BillingModelSourceResponse bills by a trusted model declaration observed
 	// in the successful upstream response. It is deliberately distinct from
 	// "upstream", which means the model sent to the provider.
-	BillingModelSourceResponse      = "response_model"
+	BillingModelSourceResponse = "response_model"
 )
 
 // Channel 渠道实体

frontend/src/i18n/locales/en/admin/channels.ts

@@ -144,8 +144,6 @@ export default {
         billingModelSourceRequested: 'Bill by requested model',
         billingModelSourceUpstream: 'Bill by final upstream model',
         billingModelSourceResponse: 'Bill by upstream response model',
-        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.',
         billingModelSourceHint: 'Controls which model name is used for pricing lookup',
         selectedCount: '{count} selected',
         searchGroups: 'Search groups...',

frontend/src/i18n/locales/zh/admin/channels.ts

@@ -144,7 +144,6 @@ export default {
         billingModelSourceRequested: '以请求模型计费',
         billingModelSourceUpstream: '以最终模型计费',
         billingModelSourceResponse: '按上游响应模型计费',
-        billingModelSourceResponseWarning: '计费基准以上游响应自报的模型为准。该模式只会降低费用、不会抬高，但请仅对可信的上游启用。',
         billingModelSourceHint: '控制使用哪个模型名称进行定价查找',
         selectedCount: '已选 {count} 个',
         searchGroups: '搜索分组...',

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 -->