[kurohune538]

Summary:

Backport the production fix from #59207 to 0.87-stable and add five regression tests, following the 0.88 backport in #59432.

The default clear NSBackgroundColorAttributeName suppresses the Japanese IME composition underline even while the native marked range remains active. Omitting that no-op attribute restores the underline. Visible backgrounds and foreground/background opacity remain intact.

This is intentionally limited to the underline root cause. It does not change TextInput delegates, selection/synchronization logic, or claim to solve every CJK composition issue discussed in #56082.

Production fix by Gregory Moskaliuk (@hryhoriiK97); regression coverage adapted from the 0.88 backport by @riottt.

Related to #48497 and the underline portion of #56082.

Changelog:

[IOS] [FIXED] - Avoid a default transparent text background that can suppress the Japanese IME composition underline.

Test Plan:

• Installed RNTester pods from source with RCT_USE_PREBUILT_RNCORE=0 bundle exec pod install --no-repo-update.
• Locally registered RCTAttributedTextUtilsTest.mm in RNTesterUnitTests for verification; that project-only edit is excluded from this PR.
• Ran RCTAttributedTextUtilsTest on an iPhone 17 Pro / iOS 26.5 simulator. The 0.87 branch has an unrelated compile failure in RCTTurboModuleArrayBufferTests.mm, so that source was excluded from this targeted run.
  • Before the production hunk: 14 passed / 4 failed (18 total).
  • After the production hunk: 18 passed / 0 failed.
• The main-branch C++/Objective-C formatting checker passes on both modified files, and git diff --check passes.
• UI evidence for the identical production hunk on 0.88: reproduction details and screenshots.

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