[iOS] Account for contentInset in Fabric maintainVisibleContentPosition autoscroll

[enahum]

Summary

The Fabric implementation of _adjustForMaintainVisibleContentPosition in RCTScrollViewComponentView.mm does not account for contentInset when computing the autoscroll threshold comparison or the scroll target position. The Paper implementation (RCTScrollView.m) already handles this correctly via bottomInset/leftInset variables. The Fabric port simply missed carrying this logic over.

Root cause

Vertical (before):

CGFloat y = _scrollView.contentOffset.y;              // ignores contentInset
[self scrollToOffset:CGPointMake(..., 0) animated:YES]; // wrong target

Vertical (after, matching Paper):

CGFloat bottomInset = [self isInverted] ? _scrollView.contentInset.top : _scrollView.contentInset.bottom;
CGFloat y = _scrollView.contentOffset.y + bottomInset; // inset-adjusted
[self scrollToOffset:CGPointMake(..., -bottomInset) animated:YES]; // correct target

Same pattern for the horizontal branch with leftInset.

Visible symptoms

On a scroll view with a non-zero contentInset:

1. Wrong threshold firing: The autoscroll threshold comparison uses raw contentOffset instead of the inset-adjusted value, causing autoscroll to fire when it should not (or fail to fire when it should).
2. Wrong scroll target: When autoscroll fires, scrollToOffset is called with 0 instead of -inset, causing the scroll view to jump to the wrong position.

The same scroll view behaves correctly on Paper (Old Architecture) since RCTScrollView.m already accounts for contentInset.

No behavior change when contentInset is zero.

Changelog:

[iOS] [Fixed] - Account for contentInset in Fabric ScrollView maintainVisibleContentPosition autoscroll threshold and target

Test Plan

• Non-inverted vertical, contentInset={{ bottom: 100 }}, autoscrollToTopThreshold: 10: insert item at index 0 while at the bottom → stays at correct position, does not jump to y=0
• Inverted vertical, contentInset={{ top: 100 }}, autoscrollToTopThreshold: 10: insert item at index 0 while at the bottom → stays at contentOffset.y = -100, does not jump to y=0
• Both cases, scrolled past the threshold: insert item → no autoscroll fires
• Non-inverted horizontal, contentInset={{ left: 100 }}: equivalent checks
• Inverted horizontal, contentInset={{ right: 100 }}: equivalent checks
• All cases with contentInset all zeros: existing behavior unchanged
• Run existing ScrollView tests: yarn jest packages/react-native/Libraries/Components/ScrollView