Negative http error codes from onHttpError.listen when website is unreachable

Is there a documentation about the negative http error codes from `onHttpError.listen`?
The goal is to lead to a custom error page on timeouts.

I simulated a non responding server by loading `https://www.google.com:81` which will never finish.
In this case, the listener receives http error code -2 on Android, and -1001 on iOS.

Can I rely on those codes to not change?

**Demo code:**
```
Widget build(BuildContext context) {
final plugin = FlutterWebviewPlugin();
plugin.onHttpError.listen((httpError) {
print('error code: ${httpError.code}, url: ${httpError.url}');
if ((Platform.isAndroid && httpError.code == '-2') ||
(Platform.isIOS && httpError.code == '-1001')) {
// navigate to custom error screen
// not sure if error codes are stable/reliable
}
});
plugin.onStateChanged.listen((state) {
print('state changed: type: ${state.type}, url: ${state.url}');
});

return WebviewScaffold(
initialChild: Center(
child: CircularProgressIndicator(),
),
url: 'https://www.google.com:81',
hidden: true,
);
}
```

**Console outputs and behaviour differ depending on the platform:**

Android:
```
state changed: type: WebViewState.startLoad, url: https://www.google.com:81/
error code: -2, url: https://www.google.com:81/
state changed: type: WebViewState.finishLoad, url: https://www.google.com:81/
```

Android finishes with a website telling about the timeout:

iOS:
```
state changed: type: WebViewState.shouldStart, url: https://www.google.com:81/
state changed: type: WebViewState.startLoad, url: https://www.google.com:81/
error code: -1001, url: ?, runtime type: WebViewHttpError
```
iOS never finishes loading and shows the progress indicator forever.

Start with the onHttpError.listen behavior shown in the demo and compare the Android and iOS outputs for the simulated unreachable website. Done means documenting whether these negative codes are stable, how platform differences should be handled, and what behavior users can rely on for custom timeout pages.

Written by the indexing model from the issue text.