Add AccessibilityInfo.isShowBordersEnabled and showBordersChanged on iOS

[camilopires]

Summary:

iOS's Show Borders setting (called Button Shapes before iOS 26.1) outlines tappable controls. UIKit and SwiftUI buttons follow it automatically, but React Native apps can't detect it, so custom Pressable buttons can't match. It can also change layout (expo/expo#48058).

This adds AccessibilityInfo.isShowBordersEnabled() and a showBordersChanged event, following the existing isDarkerSystemColorsEnabled pattern.

• iOS 26.1 deprecated UIAccessibilityButtonShapesEnabled() in favour of AXShowBordersEnabled() in the Accessibility framework.
• The module uses the new API on 26.1 and later, guarded by __IPHONE_OS_VERSION_MAX_ALLOWED for older SDKs and by @available at runtime, and the UIKit API below that.
• React-CoreModules now links the Accessibility framework, in both the podspec and Package.swift. The framework exists from iOS 14, below the 15.1 minimum, and the new symbol is weak-linked through @available.

On Android, the method resolves to false.

Discussion: react-native-community/discussions-and-proposals#1026

Changelog:

[IOS] [ADDED] - AccessibilityInfo.isShowBordersEnabled() and showBordersChanged (Show Borders, formerly Button Shapes)

Test Plan:

• yarn jest packages/react-native/Libraries/Components/AccessibilityInfo: all tests pass
• yarn flow-check: 0 errors
• yarn lint: passes
• yarn build-types --validate: API snapshot up to date
• yarn test-generated-typescript and yarn test-typescript-legacy: pass
• yarn format-check-cpp: passes
• RNTester builds for the iOS simulator: xcodebuild -workspace RNTesterPods.xcworkspace -scheme RNTester -sdk iphonesimulator, using Xcode 27.1

On an iPhone 18 Pro simulator running iOS 27.0, which uses the AXShowBordersEnabled path: I flipped the setting in the simulator's Settings app with a throwaway XCUITest, not part of this PR, and read RNTester's Accessibility → "Check if the display options are enabled" row. RNTester stayed running the whole time, so the row could only update through the change event.

• Before: "Show Borders is disabled."
• Show Borders turned on: "Show Borders is enabled."
• Turned off: "Show Borders is disabled."

Not run: iOS below 26.1, which uses the UIKit fallback. No older simulator runtime was available.

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[github-actions]

Warning

JavaScript API change detected

This PR commits an update to ReactNativeApi.d.ts, indicating a change to React Native's public JavaScript API.

• Please include a clear changelog message.
• This change will be subject to additional review.

This change was flagged as: POTENTIALLY_BREAKING

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[meta-codesync]

@javache has imported this pull request. If you are a Meta employee, you can view this in D126037985.

[meta-codesync]

@javache merged this pull request in 62a1fc1.