Fix maintainVisibleContentPosition when ScrollView content shrinks

[aryan1306]

Summary:

Fixes #58578.

When ScrollView content shrinks, layout can clamp the current scroll offset before maintainVisibleContentPosition adjusts the visible anchor. Applying the anchor delta to that clamped offset can move the list to the top.

Capture the scroll offset before mounting and use it when applying the anchor adjustment in Android and iOS Fabric. Add regression tests for vertical and horizontal shrinking content.

Changelog:

[GENERAL] [FIXED] - Preserve the visible ScrollView anchor when content shrinks.

Test Plan:

• Reproduced the jump with the linked Expo app on an Android emulator: the visible child moved from anchor 49 to child 0 after collapse.
• Ran ./gradlew :packages:react-native:ReactAndroid:testDebugUnitTest --tests com.facebook.react.views.scroll.MaintainVisibleScrollPositionHelperTest --no-daemon: all three tests passed.
• Ran the RNTesterUnitTests scheme on an iPhone 17 Pro simulator: all three tests in RCTScrollViewComponentViewTests passed, including the vertical and horizontal shrink regressions.
• Confirmed the vertical iOS regression test fails against the pre-fix code: expected offset 300, received -500.

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