[webview_flutter_wkwebview] Add support for custom URL scheme handlers

[brlumen]

Adds support for loading resources with custom URL schemes on iOS and macOS by wrapping the native WKURLSchemeHandler API.

New public API:

• WebKitWebViewControllerCreationParams.urlSchemeHandlers — a map of URL scheme to handler. Handlers are registered at creation time because WKWebViewConfiguration.setURLSchemeHandler only takes effect before the WKWebView is created.
• WebKitUrlSchemeHandler — onStart/onStop callbacks receiving a WebKitUrlSchemeTask.
• WebKitUrlSchemeTask — exposes the request url/httpMethod and the reply methods didReceiveResponse, didReceiveData, didFinish, and didFail.

Internal changes:

• New Pigeon ProxyApis: WKURLSchemeHandler (Dart-implemented, modeled after WKScriptMessageHandler) and WKURLSchemeTask; WKWebViewConfiguration.setURLSchemeHandler; constructors for the existing HTTPURLResponse, URLResponse, and NSError ProxyApis (needed to build responses from Dart).
• Calling a WKURLSchemeTask method after the web view has stopped the task raises NSException. Since the stop notification inherently races in-flight platform channel calls, the native side tracks stopped tasks (via an associated object) and turns late calls into no-ops; the equivalent race on the Dart side (stop arriving while the start handler is still reading the request) is also handled.

Tests:

• Dart unit tests for the new wrapper API.
• Native XCTests for the new ProxyApi delegates, including the stopped-task no-op guard.
• Integration tests in the example app covering: serving an image to a real WKWebView, stopping an in-flight task when the page is replaced (with late replies being no-ops), delivering a large (3 MB) response body via main frame navigation to a custom scheme URL, and 24 concurrent requests on one page.

Example use case: an email client that rewrites cid: inline-image references to a custom scheme and serves the bytes from its local attachment cache. WKWebView cannot intercept http(s): requests, so a custom scheme handler is the platform-sanctioned counterpart of Android's shouldInterceptRequest for serving locally cached subresources.

Part of flutter/flutter#27896 (implements the iOS/macOS side).

Pre-Review Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
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• I read and followed the relevant style guides and ran the auto-formatter.
• 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 linked to at least one issue that this PR fixes in the description above.
• I followed the version and CHANGELOG instructions, using semantic versioning and the repository CHANGELOG style, or I have commented below to indicate which documented exception this PR falls under¹.
• I updated/added any relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or I have commented below to indicate which test exemption this PR falls under¹.
• All existing and new tests are passing.

Footnotes

1. Regular contributors who have demonstrated familiarity with the repository guidelines only need to comment if the PR is not auto-exempted by repo tooling. ↩ ↩²

[gemini-code-assist]

Code Review

This pull request implements support for custom URL schemes in webview_flutter_wkwebview via the native WKURLSchemeHandler API, introducing WebKitUrlSchemeHandler and WebKitUrlSchemeTask along with their native Swift delegates and Pigeon bindings. The review feedback identifies a potential memory leak in _registerUrlSchemeHandler where tasks can remain in stoppedBeforeStart indefinitely, and recommends setting _stopped to true upon task completion to safeguard against post-completion calls.