[webview_flutter] Adds onHttpError callback to NavigationDelegate to catch HTTP error status codes

[HugoOlthof]

This PR adds the onHttpError callback to NavigationDelegate to catch HTTP error status codes. In our app we need to catch the status codes to show native error pages/alerts or redirect users to the native login screen if the user is unauthenticated. The existing onWebResourceError is not sufficient.

The Android implementation uses the onReceiveHttpError callback from WebViewClient and the iOS implementation uses the decidePolicyForNavigationResponse delegate from WKWebView.

Fixesflutter/flutter#39502.

Pre-launch 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.
• I read and followed the relevant style guides and ran the auto-formatter. (Unlike the flutter/flutter repo, the flutter/packages repo does use dart format.)
• 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.
• 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.

[bparrishMines]

Thanks for working on this! I made a first pass through.

[bparrishMines]

Thanks for the work on this! This looks mostly ready. I just had a few more comments about documentation.

This is also missing integration tests for Android, iOS, and the app-facing api. They're located at:
https://github.com/flutter/packages/blob/main/packages/webview_flutter/webview_flutter_android/example/integration_test/webview_flutter_test.dart

https://github.com/flutter/packages/blob/main/packages/webview_flutter/webview_flutter_wkwebview/example/integration_test/webview_flutter_test.dart

https://github.com/flutter/packages/blob/main/packages/webview_flutter/webview_flutter/example/integration_test/webview_flutter_test.dart

The test called onWebResourceError should be a good example for each file. And you should be able to use '$prefixUrl/favicon.ico' as the test url that returns a 404 status code.

I think this is ready for a secondary review since only documentation and integration tests are left.
@cyanglaz can you look over the webview_flutter_wkwebview code as a secondary reviewer?
@stuartmorgan can you look over the platform interface/app-facing code as secondary reviewer?

[stuartmorgan-g]

@HugoOlthof Are you planning on updating this based on the review feedback above?

[HugoOlthof]

@stuartmorgan Yes, I will update the PR as soon as possible!

[bparrishMines]

LGTM! with a handful of nits

After fixing the comments, you can go ahead and create the PR with just the code in webview_flutter_platform_interface.

[HugoOlthof]

This is the PR for webview_flutter_platform_interface: #3645

[HugoOlthof]

The PR for the platform implementations: #3695

[stuartmorgan-g]

Update from triage: this is waiting for the sub-PR to complete the review process.

[stuartmorgan-g]

Update from triage: sub-PR is still in progress.

[stuartmorgan-g]

Update from triage: Still waiting on #3695

[Hixie]

This is blocked on #3695 which is blocked on #4025 which is currently in review.

[stuartmorgan-g]

Update from triage: #5790 has been created as a copy of the blocking PR, to try to start unwinding this stack.

[stuartmorgan-g]

Update from triage: the last PR in the stack is resolved, and this is now blocked on getting the next PR over the line.

[stuartmorgan-g]

Update from triage: implementation PR (referenced just above) is still in progress.

[bparrishMines]

closing in favor of #6378