fix: support sticky group hierarchy toggle

[fangsmile]

Summary

• guard sticky shadow pointer events when eventArgs is missing
• forward sticky group icon clicks to toggleHierarchyState
• preserve the sticky group title position after collapse/expand

Testing

• use packages/vtable/examples/list/groupBy-oneCol.ts to reproduce issue [Bug] groupby 和 enableTreeStickCell 同时开启后，当滚动到分组标题为吸附状态时，点击分组折叠/展开无法执行折叠/展开事件 #4984
• scroll until the group title becomes sticky, then click the sticky expand/collapse icon
• verify no console error is thrown and the hierarchy state change event fires
• verify at scrollTop=284 the clicked sticky group stays near its original viewport position after collapse

Closes #4984