fix(composite): keep CN rollout on fully supported paths

- Responses WebSocket composite whitelist stays openai+grok: CN accounts
  cannot pass the WSv2 ingress transport filter and the WS HTTP bridge
  has no Responses conversion for them, so admitting CN targets only
  turns a clear policy rejection into a misleading no-available-account
- widen the responses/input_tokens and messages/count_tokens composite
  gates to the shared openai-compatible text whitelist so CN targets get
  the same local-estimate token counting as generation
- restore the Claude default-model fallback for standalone CN groups
  (admin candidates and custom models list) while keeping composite
  listings limited to CN account mapping keys
- refresh the scheduler bulk-rebuild comment and bucket capacity hints
  for the 8-platform canonical set

backend/internal/handler/gateway_handler.go

@@ -1150,7 +1150,9 @@ func (h *GatewayHandler) compositeAvailableModels(ctx context.Context, groupID *
 	for _, platform := range []string{service.PlatformAnthropic, service.PlatformGemini, service.PlatformOpenAI, service.PlatformAntigravity, service.PlatformGrok, service.PlatformKimi, service.PlatformZhipu, service.PlatformDeepseek} {
 		platformModels := h.gatewayService.GetAvailableModels(ctx, groupID, platform)
 		if len(platformModels) == 0 {
-			if _, ok := schedulablePlatforms[platform]; ok {
+			// CN 供应商没有静态默认模型列表（defaultModelIDsForPlatform 的
+			// default 分支是 Claude 列表），composite 下只暴露账号映射键。
+			if _, ok := schedulablePlatforms[platform]; ok && !service.IsCNProvider(platform) {
 				platformModels = defaultModelIDsForPlatform(platform)
 			}
 		}
@@ -1369,8 +1371,6 @@ func defaultModelIDsForPlatform(platform string) []string {
 		return mergeModelIDs(ids, nil)
 	case service.PlatformGrok:
 		return xai.DefaultModelIDs()
-	case service.PlatformKimi, service.PlatformZhipu, service.PlatformDeepseek:
-		return nil
 	case service.PlatformComposite:
 		ids := make([]string, 0)
 		seen := make(map[string]struct{})