refactor(channel): split long functions, extract shared validation, move billing validation to service

- Split Update (98→25 lines), buildCache (54→20 lines), Create (51→25 lines)
  into focused sub-functions: applyUpdateInput, checkGroupConflicts,
  fetchChannelData, populateChannelCache, storeErrorCache, getOldGroupIDs,
  invalidateAuthCacheForGroups
- Extract validateChannelConfig to eliminate duplicated validation calls
  between Create and Update
- Move validatePricingBillingMode from handler to service layer for
  proper separation of concerns
- Add error logging to IsModelRestricted (was silently swallowing errors)
- Add 12 new tests: ToUsageFields, billing mode validation, antigravity
  wildcard mapping isolation, Create/Update mapping conflict integration

backend/internal/handler/admin/channel_handler.go

@@ -1,8 +1,6 @@
 package admin
 
 import (
-	"errors"
-	"fmt"
 	"strconv"
 	"strings"
 
@@ -235,61 +233,6 @@ func pricingRequestToService(reqs []channelModelPricingRequest) []service.Channe
 	return result
 }
 
-// validatePricingBillingMode 校验计费配置
-func validatePricingBillingMode(pricing []service.ChannelModelPricing) error {
-	for _, p := range pricing {
-		// 按次/图片模式必须配置默认价格或区间
-		if p.BillingMode == service.BillingModePerRequest || p.BillingMode == service.BillingModeImage {
-			if p.PerRequestPrice == nil && len(p.Intervals) == 0 {
-				return errors.New("per-request price or intervals required for per_request/image billing mode")
-			}
-		}
-		// 校验价格不能为负
-		if err := validatePriceNotNegative("input_price", p.InputPrice); err != nil {
-			return err
-		}
-		if err := validatePriceNotNegative("output_price", p.OutputPrice); err != nil {
-			return err
-		}
-		if err := validatePriceNotNegative("cache_write_price", p.CacheWritePrice); err != nil {
-			return err
-		}
-		if err := validatePriceNotNegative("cache_read_price", p.CacheReadPrice); err != nil {
-			return err
-		}
-		if err := validatePriceNotNegative("image_output_price", p.ImageOutputPrice); err != nil {
-			return err
-		}
-		if err := validatePriceNotNegative("per_request_price", p.PerRequestPrice); err != nil {
-			return err
-		}
-		// 校验 interval：至少有一个价格字段非空
-		for _, iv := range p.Intervals {
-			if iv.InputPrice == nil && iv.OutputPrice == nil &&
-				iv.CacheWritePrice == nil && iv.CacheReadPrice == nil &&
-				iv.PerRequestPrice == nil {
-				return fmt.Errorf("interval [%d, %s] has no price fields set for model %v",
-					iv.MinTokens, formatMaxTokens(iv.MaxTokens), p.Models)
-			}
-		}
-	}
-	return nil
-}
-
-func validatePriceNotNegative(field string, val *float64) error {
-	if val != nil && *val < 0 {
-		return fmt.Errorf("%s must be >= 0", field)
-	}
-	return nil
-}
-
-func formatMaxTokens(max *int) string {
-	if max == nil {
-		return "∞"
-	}
-	return fmt.Sprintf("%d", *max)
-}
-
 // --- Handlers ---
 
 // List handles listing channels with pagination
@@ -343,10 +286,6 @@ func (h *ChannelHandler) Create(c *gin.Context) {
 	}
 
 	pricing := pricingRequestToService(req.ModelPricing)
-	if err := validatePricingBillingMode(pricing); err != nil {
-		response.ErrorFrom(c, infraerrors.BadRequest("VALIDATION_ERROR", err.Error()))
-		return
-	}
 
 	channel, err := h.channelService.Create(c.Request.Context(), &service.CreateChannelInput{
 		Name:               req.Name,
@@ -391,10 +330,6 @@ func (h *ChannelHandler) Update(c *gin.Context) {
 	}
 	if req.ModelPricing != nil {
 		pricing := pricingRequestToService(*req.ModelPricing)
-		if err := validatePricingBillingMode(pricing); err != nil {
-			response.ErrorFrom(c, infraerrors.BadRequest("VALIDATION_ERROR", err.Error()))
-			return
-		}
 		input.ModelPricing = &pricing
 	}