docs: document what find() can and cannot see (#236) (CP: 25.3)

[vaadin-bot]

This PR cherry-picks changes from the original PR #236 to branch 25.3.

Original PR description

Summary

Corrects the documentation of what find() can reach. A closed context menu is not attached to the UI, so its items cannot be clicked — the old Javadoc and README said open() was optional, which was wrong. This also names the grid context menu tester as the way in for grid menus.

Fixes #218

What changed

This is a documentation-only change. No code behaviour changes, and nothing becomes incompatible — the runtime already threw IllegalStateException for these calls; only the docs were wrong.

• ContextMenuTester: the class and method Javadoc no longer claim that open() is optional. It now states that clickItem(...), isItemChecked(...) and getItemTooltipText(...) need the menu to be open and otherwise throw IllegalStateException. The @throws IllegalStateException tags list the closed menu as a cause. Only the tester-scoped find(Class) works on a closed menu, and it returns detached components.
• ComponentQuery: adds a short note that a query walks the server-side component tree, and that per-item rendered components and closed overlay content are not in it. Since a query returns an empty result instead of failing, such a component reads as if it was never created.
• README: rewrites the closed-menu paragraph to match the real behaviour and adds a paragraph on GridContextMenu, whose tester takes a row (test(grid).contextMenu(row), GridContextMenuTester.open(row)).
• find(Class) entry points (BaseBrowserlessTest, BrowserlessUIContext, ComponentTester, AbstractBrowserlessExtension): the pointers now name GridTester.contextMenu(row).open() for grid context menus, because GridContextMenu is not a ContextMenu and ContextMenuTester does not apply to it.

Test summary

• A tester-scoped find(...) on a context menu that was never opened finds the items and returns them detached; after open() the same items are attached.
• A top-level find(...) does not see the component a grid component column renders per row, confirming that such components are not part of the component tree.