[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.

To align with the repository's contribution guidelines, please update the pull request title to include the package name (e.g., [webview_flutter_wkwebview] Fix crash in tests) and fill out the PR description with the necessary details, including the checklist and a link to the issue this PR resolves.

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()
  }