[webview_flutter_wkwebview] Fixes crash when running WKNavigationResponse tests on Xcode 26

[bparrishMines]

Fixes flutter/flutter#173326

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[gemini-code-assist]

Code Review

This pull request addresses a crash that occurs during testing by introducing a singleton pattern for TestNavigationResponse. By using a static instance, it prevents premature deallocation of WKNavigationResponse objects within the test environment, which was causing the crash. The changes are correctly applied across the test files.

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I have one suggestion to make the singleton implementation more robust.

[gemini-code-assist]

To fully enforce the singleton pattern and prevent accidental creation of new TestNavigationResponse instances elsewhere in the tests, it's a good practice to make the initializer private. This will prevent reintroducing the deallocation crash in the future.

  static let instance = TestNavigationResponse()

  private override init() {
    super.init()
  }

[LongCatIsLooong]

I'm guessing this is only deferring the crash to when the program terminates (instead of at the end of the test), now that the instance is made static.

The documentation says

You don’t create WKNavigationResponse objects directly.

So I guess that explains why it is tricky to create WKNavigationResponse mocks. Someone actually had the exact same crash stacktrace here: https://developer.apple.com/forums/thread/804615, and they said they were able to workaround the crash (without preventing the mocks from deallocating).