[webview_flutter_wkwebview] Prevents test WKFrameInfo from deallocating during tests to prevent crash

[bparrishMines]

Fixes flutter/flutter#182846

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Code Review

This pull request refactors unit tests in the WKWebView package to use static instances of TestFrameInfo and TestFrameInfoWithNilRequest to prevent crashes during deallocation on newer iOS versions. The review feedback identifies a recurring typo in the code comments where 'iOS 26+' is cited instead of the correct iOS version.

[bparrishMines]

@okorohelijah @vashworth I believe this fixes flutter/flutter#182846, so feel free to try or use this when upgrading CI to iOS 26. The tests pass when I run it locally on iOS 26 with Xcode 26.4 with this change.