fix: support context menu on canvas blank area

[fangsmile]

Summary

• Fix [Feature] 右键菜单插件（@visactor/vtable-plugins 中的 ContextMenuPlugin）不能在整个画布中触发 #5215: support ContextMenuPlugin on canvas blank area when contextMenuWorkOnlyCell=false.
• Add plugin init lifecycle so the option is applied before first render.
• Add a dedicated plugin demo: context-menu/issue-5215-context-menu-canvas.

Validation

• PASS: packages/vtable-plugins rushx compile.
• BLOCKED: normal push hook is blocked by local Rush link state: Link flag invalid. Did you run rush install or rush update?

Fixes #5215