[webview_flutter] Fix crash in iOS external native API

[stuartmorgan-g]

Fixes a crash if FWFWebViewFlutterWKWebViewExternalAPI's webView(forIdentifier:withPluginRegistry:) is called with a registry that doesn't contain a published webview plugin value. This can happen if the wrong object is passed in (e.g., the FlutterAppDelegate in an app that has adopted UIScene).

Fixes flutter/flutter#181865

Pre-Review Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
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• I linked to at least one issue that this PR fixes in the description above.
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• 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.
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Footnotes

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[gemini-code-assist]

Code Review

This pull request addresses a crash in the iOS external native API by replacing a force-cast with a safe optional cast. This correctly handles cases where the webview plugin isn't found in the provided registry. The change is accompanied by a new unit test to verify the fix, along with corresponding updates to the changelog and package version. The implementation is sound, but I've noted a minor issue in the new test code that will cause a compilation error.

[gemini-code-assist]

The registrar(forPlugin:) method for macOS in EmptyRegistry is declared to return a non-optional FlutterPluginRegistrar, but the implementation returns nil. This will cause a compilation error on macOS because the protocol requires a non-optional return value.

To resolve this, you can return the registrar property, which will satisfy the protocol requirement without altering the test's behavior.

Suggested change

	func registrar(forPlugin pluginKey: String) -> FlutterPluginRegistrar {
	return nil
	}
	func registrar(forPlugin pluginKey: String) -> FlutterPluginRegistrar {
	return registrar
	}

[bparrishMines]

LGTM