fix: find elements and components inside multi-root functional components

[ValentinYoushkevich]

Follow-up to #2568 / #2579.

findComponent(Func) on a functional component returning an array gives a DOMWrapper around the fragment anchor, so:

• html()/text() lose nested arrays and slots
• find/findAll/get find nothing
• findComponent/findAllComponents find nothing

const Func: FunctionalComponent = (_, { slots }) => [h('h1', 'title'), slots.default?.()]
const func = mount({ components: { Func }, template: '<div><Func><span>foo</span></Func></div>' }).findComponent(Func)
func.html()                // '<h1>title</h1>\n' (span missing)
func.find('span').exists() // false

The wrapper now collects its roots with getRootNodes() (like VueWrapper) and resolves the functional instance from the fragment anchor, reading its current subTree so results stay in sync after a re-render.

Note: like a multi-root VueWrapper, html() no longer prints <!----> for empty (null/false) roots. isVisible()/classes()/attributes()/trigger() still target the anchor.

Repro on 2.5.1: playground

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