fix(ops): 默认忽略 count_tokens 404 错误

将 IgnoreCountTokensErrors 默认值从 false 改为 true。

count_tokens 返回 404 是预期业务行为（上游不支持 endpoint，
客户端应 fallback 到本地 tokenizer 估算），不应被视为错误。
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/internal/service/ops_settings.go

@@ -368,7 +368,7 @@ func defaultOpsAdvancedSettings() *OpsAdvancedSettings {
 		Aggregation: OpsAggregationSettings{
 			AggregationEnabled: false,
 		},
-		IgnoreCountTokensErrors:   false,
+		IgnoreCountTokensErrors:   true,   // count_tokens 404 是预期行为，默认忽略
 		IgnoreContextCanceled:     true,  // Default to true - client disconnects are not errors
 		IgnoreNoAvailableAccounts: false, // Default to false - this is a real routing issue
 		AutoRefreshEnabled:        false,