[breadcrumbs] Task 2: BreadcrumbsItem — constructors, text, path, prefix

[web-padawan]

Implementation task 2 of the Breadcrumbs Flow component (spec-driven development pipeline).

What to do:

• Implement the seven BreadcrumbsItem constructors mirroring SideNavItem's overload set: (text), (text, String path), (text, Class<? extends Component> view), (text, Class, RouteParameters), plus the three prefix-component variants ending in Component prefixComponent
• Implement the path accessors: getPath(), setPath(String) (reflects to the path attribute; setPath(null) removes it), setPath(Class<? extends Component>), setPath(Class, RouteParameters) — the class overloads resolve via RouteConfiguration.forRegistry(ComponentUtil.getRouter(this).getRegistry()).getUrl(view, params) exactly like SideNavItem
• Verify the inherited HasText / HasEnabled / HasPrefix surface works (text content, disabled attribute reflection, prefix slot)
• No events, no @Synchronize'd properties

Spec references:

• flow-tasks.md — Task 2
• flow-spec.md

Part of vaadin/web-components#7960