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_test.go

@@ -400,103 +400,3 @@ func TestPricingRequestToService_NilPriceFields(t *testing.T) {
 	require.Nil(t, r.ImageOutputPrice)
 	require.Nil(t, r.PerRequestPrice)
 }
-
-// ---------------------------------------------------------------------------
-// 3. validatePricingBillingMode
-// ---------------------------------------------------------------------------
-
-func TestValidatePricingBillingMode(t *testing.T) {
-	tests := []struct {
-		name    string
-		pricing []service.ChannelModelPricing
-		wantErr bool
-	}{
-		{
-			name: "token mode - valid",
-			pricing: []service.ChannelModelPricing{
-				{BillingMode: service.BillingModeToken},
-			},
-			wantErr: false,
-		},
-		{
-			name: "per_request with price - valid",
-			pricing: []service.ChannelModelPricing{
-				{
-					BillingMode:     service.BillingModePerRequest,
-					PerRequestPrice: float64Ptr(0.5),
-				},
-			},
-			wantErr: false,
-		},
-		{
-			name: "per_request with intervals - valid",
-			pricing: []service.ChannelModelPricing{
-				{
-					BillingMode: service.BillingModePerRequest,
-					Intervals: []service.PricingInterval{
-						{MinTokens: 0, MaxTokens: intPtr(1000), PerRequestPrice: float64Ptr(0.1)},
-					},
-				},
-			},
-			wantErr: false,
-		},
-		{
-			name: "per_request no price no intervals - invalid",
-			pricing: []service.ChannelModelPricing{
-				{BillingMode: service.BillingModePerRequest},
-			},
-			wantErr: true,
-		},
-		{
-			name: "image with price - valid",
-			pricing: []service.ChannelModelPricing{
-				{
-					BillingMode:     service.BillingModeImage,
-					PerRequestPrice: float64Ptr(0.2),
-				},
-			},
-			wantErr: false,
-		},
-		{
-			name: "image no price no intervals - invalid",
-			pricing: []service.ChannelModelPricing{
-				{BillingMode: service.BillingModeImage},
-			},
-			wantErr: true,
-		},
-		{
-			name:    "empty list - valid",
-			pricing: []service.ChannelModelPricing{},
-			wantErr: false,
-		},
-		{
-			name: "mixed modes with invalid image - invalid",
-			pricing: []service.ChannelModelPricing{
-				{
-					BillingMode: service.BillingModeToken,
-					InputPrice:  float64Ptr(0.01),
-				},
-				{
-					BillingMode:     service.BillingModePerRequest,
-					PerRequestPrice: float64Ptr(0.5),
-				},
-				{
-					BillingMode: service.BillingModeImage,
-				},
-			},
-			wantErr: true,
-		},
-	}
-
-	for _, tt := range tests {
-		t.Run(tt.name, func(t *testing.T) {
-			err := validatePricingBillingMode(tt.pricing)
-			if tt.wantErr {
-				require.Error(t, err)
-				require.Contains(t, err.Error(), "per-request price or intervals required")
-			} else {
-				require.NoError(t, err)
-			}
-		})
-	}
-}