fix(VSlideGroup): check hasAffixes in hasNext computed property

[w-daniels]

fixes #21115

Description

Fixes issue #21115 where hasNext computed property in VSlideGroup only checks if containerRef.value exists before determining whether to enable the next affix. However, it doesn't take into account whether arrows/affixes should be displayed at all based on the current hasAffixes state (which can be simulated by resizing the components container, leading to the issue).

Changes

Modified hasNext computed property to return false when affixes aren't visible:

const hasNext = computed(() => {
  if (!containerRef.value || !hasAffixes.value) return false
  // ... rest 
})

Markup

see playground