update pigeon in webview

[bparrishMines]

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Code Review

This pull request updates the pigeon dependency to version 26.0.0 across webview_flutter_android and webview_flutter_wkwebview packages. The corresponding generated files have been updated, introducing a new pattern with PigeonOverrides to improve testability by allowing mocking of pigeon-generated classes. The manual adjustment in webview_android_widget.dart to use super.pigeon_new is correct and necessary due to these changes.

The changes look good and are consistent with a pigeon version upgrade.

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