Fix iOS crash when truncating composed text

[gooddev97]

Summary:

Fixes #57499.

processTruncatedAttributedText updated color attributes while NSLayoutManager was enumerating line fragments. When the truncation boundary landed inside a composed character sequence, TextKit attribute fixing could invalidate the glyph-to-character mapping and later raise NSRangeException.

This change:

• records the truncated character range during line enumeration and applies attributes afterward in one edit transaction;
• clamps and expands the range to full composed character sequences;
• clamps highlighted-character enumeration to the current text storage length;
• adds regression coverage for mutation timing/range handling and the Unicode width-sweep reproducer from the issue.

Changelog:

[IOS] [FIXED] - Prevent crashes when text truncation splits a composed character sequence.

Test Plan:

• yarn clang-format packages/react-native/ReactCommon/react/renderer/textlayoutmanager/platform/ios/react/renderer/textlayoutmanager/RCTTextLayoutManager.mm packages/react-native/React/Tests/Text/RCTTextLayoutManagerTests.mm — passed.
• Temporarily enabled the React-RCTFabric/Tests testspec in the local RNTester Podfile (not part of this change), then ran:

xcodebuild test -quiet \
  -workspace RNTesterPods.xcworkspace \
  -scheme React-RCTFabric-Unit-Tests \
  -sdk iphonesimulator \
  -destination "platform=iOS Simulator,id=B3D3DA0E-E608-4CBE-A8FC-F5A0C328629B" \
  -derivedDataPath /tmp/RCTFabricTextLayoutBuild \
  -only-testing:React-RCTFabric-Unit-Tests/RCTTextLayoutManagerTests \
  EXCLUDED_SOURCE_FILE_NAMES="RCTAttributedTextUtilsTest.mm RCTParagraphComponentViewTests.mm RCTTextInputUtilsTest.mm RCTComponentViewRegistryTests.mm RCTScrollViewComponentViewTests.mm RCTViewComponentViewTests.mm RCTViewComponentViewUnmountTests.mm" \
  LIBRARY_SEARCH_PATHS="$(inherited) /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib/swift/iphonesimulator"

Result: both RCTTextLayoutManagerTests tests passed on an iPhone 17 Pro simulator running iOS 26.5. The targeted regression test was also verified to fail before the fix and pass afterward.