[RN] Fix VirtualizedSectionList.scrollToLocation off-by-one and sticky header offset

Summary:
VirtualizedSectionList.scrollToLocation incorrectly mapped itemIndex to the underlying flat list index. It used itemIndex directly instead of itemIndex + 1 to skip the section header, so itemIndex: 0 landed on the header and every other item was one row too early. It also only added the sticky-header viewOffset compensation when itemIndex > 0, so the first item was rendered under the sticky header.

This change adds the missing + 1 for the header row and makes the sticky-header offset unconditional, using the section header measured height (index - itemIndex - 1). This is a breaking behavioural fix with no type change.

Thanks to Marc Rousavy (mrousavy) for the diagnosis: #50143

Changelog:
[General][Breaking] - Fix SectionList and VirtualizedSectionList scrollToLocation to correctly account for section headers and sticky headers.

Test Plan:
- js1 test xplat/js/react-native-github/packages/virtualized-lists/Lists/__tests__/VirtualizedSectionList-test.js — 18 passed
- arc lint clean

Differential Revision: https://www.internalfb.com/intern/phabricator/D118547738

Author: Amro Altahtamouni

packages/virtualized-lists/Lists/VirtualizedSectionList.js

@@ -138,7 +138,7 @@ class VirtualizedSectionList<
   State,
 > {
   scrollToLocation(params: ScrollToLocationParamsType) {
-    let index = params.itemIndex;
+    let index = params.itemIndex + 1;
     for (let i = 0; i < params.sectionIndex; i++) {
       index += this.props.getItemCount(this.props.sections[i].data) + 2;
     }
@@ -147,10 +147,10 @@ class VirtualizedSectionList<
       return;
     }
     const listRef = this._listRef;
-    if (params.itemIndex > 0 && this.props.stickySectionHeadersEnabled) {
+    if (this.props.stickySectionHeadersEnabled) {
       const frame = listRef
         .__getListMetrics()
-        .getCellMetricsApprox(index - params.itemIndex, listRef.props);
+        .getCellMetricsApprox(index - params.itemIndex - 1, listRef.props);
       viewOffset += frame.length;
     }
     const toIndexParams: {