fix(ios): restore VoiceOver increment/decrement on Fabric

[kosmydel]

Summary:

On Fabric, RNCSliderComponentView hosts UISlider as contentView. RCTViewComponentView treats that as the accessibility leaf, so VoiceOver focuses the wrapper instead of the slider. The wrapper is a plain UIView and does not implement increment/decrement, which drops UISlider's Adjustable trait.

This matches React Native's own text-input pattern: the host is not an accessibility element, and accessibilityElement points at the inner control.

Test Plan:

1. New Architecture iOS app with a Slider that has an accessibilityLabel (no custom accessibilityValue).
2. Enable VoiceOver. Swipe to the slider.
3. Before: focus is a generic view; swipe up/down does not change the value.
4. After: VoiceOver reports Adjustable; swipe up increments and swipe down decrements; the spoken value updates.

Expo example App.tsx

import {useState} from 'react';
import {StatusBar} from 'expo-status-bar';
import {StyleSheet, Text, View} from 'react-native';
import Slider from '@react-native-community/slider';

const MIN = 0;
const MAX = 10;
const STEP = 1;

// VoiceOver uses index = slider value. Length must be maximumValue + 1.
const ACCESSIBILITY_INCREMENTS = Array.from({length: MAX - MIN + 1}, (_, i) =>
  String((MIN + i) * 10),
);

export default function App() {
  const [value, setValue] = useState(5);

  return (
    <View style={styles.container}>
      <Text style={styles.title} accessibilityRole="header">
        Slider draft
      </Text>
      <Text
        style={styles.value}
        importantForAccessibility="no"
        accessibilityElementsHidden>
        {value * 10}%
      </Text>
      <Slider
        style={styles.slider}
        minimumValue={MIN}
        maximumValue={MAX}
        step={STEP}
        value={value}
        onValueChange={setValue}
        tapToSeek
        minimumTrackTintColor="#1411AB"
        maximumTrackTintColor="#ABABAB"
        thumbTintColor="#1411AB"
        thumbSize={32}
        accessibilityLabel="Volume"
        accessibilityHint="Swipe up or down with VoiceOver to adjust"
        accessibilityRole="adjustable"
        accessibilityUnits="percents"
        accessibilityIncrements={ACCESSIBILITY_INCREMENTS}
      />
      <StatusBar style="auto" />
    </View>
  );
}

const styles = StyleSheet.create({
  container: {
    flex: 1,
    backgroundColor: '#fff',
    alignItems: 'center',
    justifyContent: 'center',
    paddingHorizontal: 24,
  },
  title: {
    fontSize: 24,
    marginBottom: 12,
  },
  value: {
    fontSize: 18,
    marginBottom: 24,
  },
  slider: {
    width: '100%',
    height: 40,
  },
});