[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.

[okorohelijah]

@bparrishMines Thanks for taking a look! I was able to test these changes on CI PR #10635 and can confirm they resolve the crash.

However a separate integration test failure appeared in NavigationDelegate onWebResourceError. It looks like on iOS 26+, NSError.userInfo no longer includes the NSURLErrorFailingURLStringKey for DNS failures, causing error.url to be null and failing the current assertion:
expect(error.url?.startsWith('https://www.notawebsite..com'), isTrue)

Since the fix is just a small adjustment (checking the URL only if it's non-null), I thought it might be worth including here

Would you be open to including that fix in this PR, or would you prefer I keep it separate? wdyt?

Build: https://ci.chromium.org/ui/p/flutter/builders/try/Mac_arm64%20ios_platform_tests_shard_5%20master/28011/overview

log: https://logs.chromium.org/logs/flutter/buildbucket/cr-buildbucket/8685256132893502657/+/u/Run_package_tests/drive_examples/stdout?format=raw