Map role="slider" to the adjustable accessibility trait on iOS (#59463)

Summary:
Hi, I'm a React Native developer. Thanks for the recent iOS accessibility work.

On iOS, `role="slider"` ends up with no accessibility traits, so VoiceOver doesn't treat the view as adjustable. `accessibilityRole="adjustable"` on the same view works. The traits come from `fromString(const std::string &, AccessibilityTraits &)` in `accessibilityPropsConversions.h`, which has an `"adjustable"` case but no `"slider"` case.

Before the ARIA roles moved to native in #37309, the JS mapping turned `role: 'slider'` into `accessibilityRole: 'adjustable'`, so this used to work. The native table maps `slider` to no trait instead. This adds `slider` to the `adjustable` case, the same way `img`/`image` and `heading`/`header` share a case. Android doesn't read `accessibilityTraits`, so only iOS changes.

It only touches `slider`. The other roles listed in #50123 stay as they are, since the comment above `RCTViewIsInteractiveAccessibilityElement` says some of them carry no trait on purpose. I also left the legacy `RCTViewManager.m` table alone, which maps `slider` to `UIAccessibilityTraitNone` as well. Happy to change that one too if it's still wanted.

Part of #50123.

## Changelog:

[IOS] [FIXED] - `role="slider"` now gets the adjustable accessibility trait, like `accessibilityRole="adjustable"`

Pull Request resolved: #59463

Test Plan:
RNTester on an iPhone 18 Pro simulator (iOS 27.0), with the Playground rendering:

```tsx
<View accessible role="slider" accessibilityLabel="Role slider" accessibilityValue={{min: 0, max: 10, now: 5}} />
<View accessible accessibilityRole="adjustable" accessibilityLabel="AccessibilityRole adjustable" accessibilityValue={{min: 0, max: 10, now: 5}} />
```

The simulator's accessibility tree for both views:

```
main     Role slider                   AXGroup       traits: none
         AccessibilityRole adjustable  AXAdjustable  traits: adjustable
this PR  Role slider                   AXAdjustable  traits: adjustable
         AccessibilityRole adjustable  AXAdjustable  traits: adjustable
```

- Added `derives_adjustable_trait_from_slider_role` to `AccessibilityPropsTest.cpp`, next to `derives_traits_from_role`. I couldn't run it, because I didn't find a way to run the ReactCommon gtests from the open-source checkout.
- `node scripts/clang-format.js --check` passes on both files.
- Not checked: the legacy architecture.

Reviewed By: christophpurrer

Differential Revision: D127482367

Pulled By: Abbondanzo

fbshipit-source-id: bd315b64e1492c92d7a2b20831bab2dd636d2360

Author: jeltehomminga

packages/react-native/ReactCommon/react/renderer/components/view/accessibilityPropsConversions.h

@@ -71,7 +71,7 @@ inline void fromString(const std::string &string, AccessibilityTraits &result)
     result = AccessibilityTraits::StartsMediaSession;
     return;
   }
-  if (string == "adjustable") {
+  if (string == "adjustable" || string == "slider") {
     result = AccessibilityTraits::Adjustable;
     return;
   }

packages/react-native/ReactCommon/react/renderer/components/view/tests/AccessibilityPropsTest.cpp

@@ -44,6 +44,13 @@ TEST(AccessibilityPropsTest, derives_traits_from_role) {
   EXPECT_TRUE(hasTrait(props.accessibilityTraits, AccessibilityTraits::Button));
 }
 
+TEST(AccessibilityPropsTest, derives_adjustable_trait_from_slider_role) {
+  auto props = parse(folly::dynamic::object("role", "slider"));
+
+  EXPECT_TRUE(
+      hasTrait(props.accessibilityTraits, AccessibilityTraits::Adjustable));
+}
+
 TEST(AccessibilityPropsTest, derives_traits_from_accessibility_role) {
   auto props = parse(folly::dynamic::object("accessibilityRole", "image"));