Add list size tracking for production issue diagnostics

[xiaoyue26]

This commit introduces apt_list_size(), a critical diagnostic function
to address actual production incidents where object accumulation has led
to resource depletion. The function provides real-time visibility into
apt_obj_list_t element counts, enabling:

1. Early detection of abnormal list growth patterns
2. Proactive monitoring to prevent resource exhaustion
3. Root cause analysis for memory accumulation issues
4. Threshold-based alerting before system degradation occurs