refactor(channel): 抽取渠道映射公共函数 + OpenAI映射到body + 空响应修复 + 清理日志

- 抽取 ResolveChannelMappingAndRestrict 统一入口（5处→1个方法）
- 抽取 BuildModelMappingChain 到 ChannelMappingResult 方法（5处→1行调用）
- OpenAI 三入口 Forward 前应用渠道映射到请求体
- OpenAI Responses/Messages 限制检查添加错误响应
- 清理前端 3 处 console.log 调试日志

frontend/src/views/admin/ChannelsView.vue

@@ -683,7 +683,6 @@ function formToAPI(): { group_ids: number[], model_pricing: ChannelModelPricing[
 
     // Model pricing with platform tag
     for (const entry of section.model_pricing) {
-      console.log('[formToAPI] entry:', JSON.stringify({ models: entry.models, billing_mode: entry.billing_mode, per_request_price: entry.per_request_price }))
       if (entry.models.length === 0) continue
       model_pricing.push({
         platform: section.platform,
@@ -700,7 +699,6 @@ function formToAPI(): { group_ids: number[], model_pricing: ChannelModelPricing[
     }
   }
 
-  console.log('[formToAPI] result:', JSON.stringify({ group_ids, model_pricing_count: model_pricing.length, model_mapping_keys: Object.keys(model_mapping), platforms_count: form.platforms.length, pricing_entries: form.platforms.map(s => s.model_pricing.length) }))
   return { group_ids, model_pricing, model_mapping }
 }
 
@@ -883,7 +881,6 @@ async function handleSubmit() {
   }
 
   const { group_ids, model_pricing, model_mapping } = formToAPI()
-  console.log('[handleSubmit] model_pricing to send:', JSON.stringify(model_pricing))
 
   submitting.value = true
   try {