feat(VDataTableVirtual): expose containerRef for scroll container access

[mixelburg]

Description

Resolves #22647

VDataTableVirtual manages its own internal scroll container (.v-table__wrapper), but the element was not accessible through any public API. This made implementing reliable infinite scroll or custom scroll detection unnecessarily brittle — users had to reach into internals via querySelector('.v-table__wrapper') which can break at any time.

This PR exposes containerRef in the component's public API (alongside the existing calculateVisibleItems and scrollToIndex).

Usage

<template>
  <v-data-table-virtual ref="table" :items="items" :headers="headers" height="400">
    <template #tfoot>
      <div ref="sentinel" />
    </template>
  </v-data-table-virtual>
</template>

<script setup>
const table = ref()
const sentinel = ref()

onMounted(() => {
  const observer = new IntersectionObserver(([entry]) => {
    if (entry.isIntersecting) loadMore()
  }, { root: table.value.containerRef })

  observer.observe(sentinel.value)
})
</script>

Changes

• Exposed containerRef in VDataTableVirtual's return value (matches how VVirtualScroll already works)
• Added API doc entry in VDataTableVirtual.json