fix(channels): supported models = mapping ∪ pricing with global LiteLLM fallback

Why: channels with model pricing entries but no model mapping (e.g. azcc with
3 priced claude models, no mapping) were rendering as 未配置模型 in the
'Available Channels' page. The algorithm only iterated ModelMapping and
silently dropped any platform without a mapping entry.

Changes:
- channel.go: SupportedModels now unions mapping + pricing entries.
  For exact mapping src → target, pricing is looked up by target (the actually
  billed name), not by src.
- channel_available.go: ListAvailable enriches each entry with nil pricing
  via PricingService.GetModelPricing (global LiteLLM fallback) so the popover
  always shows a price.
- channel_service.go: NewChannelService takes *PricingService as 4th param.
- channel_test.go: rewrote 4 tests that froze the old mapping-only semantics;
  added pricing-only / mapping-target / target-missing coverage.

backend/internal/service/channel_available.go

@@ -36,7 +36,10 @@ type AvailableChannel struct {
 
 // ListAvailable 返回所有渠道的可用视图：每个渠道附带关联分组信息与支持模型列表。
 //
-// 支持模型通过 (*Channel).SupportedModels() 计算得到（见 channel.go）。
+// 支持模型通过 (*Channel).SupportedModels() 计算（mapping ∪ pricing 并联）。
+// 对于渠道未配置定价的模型，进一步用 PricingService 的全局 LiteLLM 数据合成
+// 一份展示用定价，让用户看到默认价格而非"未配置"。
+//
 // 关联分组信息通过 groupRepo.ListActive 查询后按 ID 映射；渠道 GroupIDs 中未在活跃列表中
 // 的分组（已停用或删除）会被忽略。
 //
@@ -78,6 +81,9 @@ func (s *ChannelService) ListAvailable(ctx context.Context) ([]AvailableChannel,
 
 		ch.normalizeBillingModelSource()
 
+		supported := ch.SupportedModels()
+		s.fillGlobalPricingFallback(supported)
+
 		out = append(out, AvailableChannel{
 			ID:                 ch.ID,
 			Name:               ch.Name,
@@ -86,7 +92,7 @@ func (s *ChannelService) ListAvailable(ctx context.Context) ([]AvailableChannel,
 			BillingModelSource: ch.BillingModelSource,
 			RestrictModels:     ch.RestrictModels,
 			Groups:             groups,
-			SupportedModels:    ch.SupportedModels(),
+			SupportedModels:    supported,
 		})
 	}
 
@@ -95,3 +101,49 @@ func (s *ChannelService) ListAvailable(ctx context.Context) ([]AvailableChannel,
 	})
 	return out, nil
 }
+
+// fillGlobalPricingFallback 对未命中渠道定价的支持模型，从全局 LiteLLM 数据合成一份
+// 展示用定价（按 token 计费）。仅用于「可用渠道」展示，不影响真实计费链路。
+//
+// 当 s.pricingService 为 nil（测试场景），跳过回落。
+func (s *ChannelService) fillGlobalPricingFallback(models []SupportedModel) {
+	if s.pricingService == nil {
+		return
+	}
+	for i := range models {
+		if models[i].Pricing != nil {
+			continue
+		}
+		lp := s.pricingService.GetModelPricing(models[i].Name)
+		if lp == nil {
+			continue
+		}
+		models[i].Pricing = synthesizePricingFromLiteLLM(lp)
+	}
+}
+
+// synthesizePricingFromLiteLLM 把 LiteLLM 的定价数据转成 ChannelModelPricing 形态，
+// 仅用于展示。BillingMode 固定为 token；图片场景的 OutputCostPerImageToken 也归到
+// ImageOutputPrice 字段（与渠道侧"图片输出按 token 计价"语义一致）。
+//
+// LiteLLM 中字段 0 视为未配置，不带入展示。
+func synthesizePricingFromLiteLLM(lp *LiteLLMModelPricing) *ChannelModelPricing {
+	if lp == nil {
+		return nil
+	}
+	return &ChannelModelPricing{
+		BillingMode:      BillingModeToken,
+		InputPrice:       nonZeroPtr(lp.InputCostPerToken),
+		OutputPrice:      nonZeroPtr(lp.OutputCostPerToken),
+		CacheWritePrice:  nonZeroPtr(lp.CacheCreationInputTokenCost),
+		CacheReadPrice:   nonZeroPtr(lp.CacheReadInputTokenCost),
+		ImageOutputPrice: nonZeroPtr(lp.OutputCostPerImageToken),
+	}
+}
+
+func nonZeroPtr(v float64) *float64 {
+	if v == 0 {
+		return nil
+	}
+	return &v
+}