fix(v-intersect): auto-detect scrollable ancestor as root

[VaishnaviD5900]

Description

Fixes #22147

When using v-intersect on a VTimelineItem (or any element inside
a scrollable container), the IntersectionObserver root was always null,
causing it to use the viewport instead of the nearest scrollable ancestor.

Root Cause

The mounted function in the intersect directive created the
IntersectionObserver with empty options {}, defaulting root to null.
Additionally, the DOM wasn't fully available at mount time.

Fix

• Added getScrollableAncestor() helper that walks up the DOM to find
  the nearest scrollable ancestor
• Wrapped observer creation in nextTick to ensure DOM is fully mounted
• When no explicit root is provided, auto-detect the nearest scrollable
  ancestor as the root

How to Test

1. Place v-intersect on a VTimelineItem inside a scrollable container
2. Verify observer.root is the scrollable container, not null
3. Verify isIntersecting correctly reflects visibility within the container

Closes

Closes #22147