[webview_flutter_wkwebview] Regenerate with pigeon 29.0.2

[Xelorium]

Description

Regenerates webview_flutter_wkwebview with pigeon 29.0.2, which contains the Swift generator fix from #12531.

The generated WKNavigationDelegate.didReceiveAuthenticationChallenge reply handler now treats NSNull as a null reply, so a response the native PigeonInstanceManager can no longer resolve reports a null-error instead of force-casting and aborting the process:

Could not cast value of type 'NSNull' (0x...) to 'webview_flutter_wkwebview.AuthenticationChallengeResponse' (0x...).

The diff is large because the package was last generated with 27.3.2, so it also picks up the 28.x/29.x generator changes; no hand-written code needed updating.

Verification

• flutter analyze is clean.
• All 154 unit tests pass.
• The iOS example builds for the simulator.
• The NSNull check is present at the AuthenticationChallengeResponse call site in the regenerated WebKitLibrary.g.swift.

Fixes flutter/flutter#191254

Pre-Review Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
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• I signed the CLA.
• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I listed at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or this PR is exempt from version changes.
• I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style, or this PR is [exempt from CHANGELOG changes].
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• All existing and new tests are passing.

[flutter-dashboard]

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[gemini-code-assist]

Code Review

This pull request updates the webview_flutter_wkwebview package version to 3.26.2 and upgrades the pigeon dependency to ^29.0.2 to resolve a crash related to WKNavigationDelegate authentication challenges. I have no feedback to provide as there were no review comments.