[webview_flutter_web] Add WebWebViewController.runJavaScript

[sophiebremer]

This PR adds an implementation for WebWebViewController.runJavaScript and changes the way WebWebViewController.loadHtmlString injects HTML code while also adding support for the baseUrl option.

The following issues are fixed:

• [webview_flutter] Support runJavaScript for Web when the content has the same origin flutter#173899

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[gemini-code-assist]

Code Review

This pull request introduces the runJavaScript method for the web implementation of WebView and refactors loadHtmlString to better support the baseUrl parameter. The changes are generally good, but there are a few areas for improvement regarding event listener management and robustness against race conditions. I've identified a potential memory leak in loadHtmlString and a high-severity issue in _getIFrameDocument that could lead to an infinite loop. I've provided suggestions to address these issues by using one-time event listeners and replacing polling with a more robust event-based approach.

[gemini-code-assist]

The load event listener is added every time loadHtmlString is called but is never removed. This can lead to a memory leak if this method is called multiple times on the same controller, as multiple listeners will be attached.

To ensure the listener is only called once per loadHtmlString call, you can use the once option in addEventListener. This will automatically remove the listener after it has been invoked.

    _webWebViewParams.iFrame.addEventListener(
      'load',
      (web.Event _) {
        _webWebViewParams.iFrame.contentDocument?.write(html.toJS);
        loading.complete();
      }.toJS,
      AddEventListenerOptions(once: true),
    );

[stuartmorgan-g]

From triage: @sophiebremer Are you planning on updating this PR to have graceful fallback to a clear error when run against anything other than a same-origin iframe, instead of crashing and/or hanging?

[sophiebremer]

@stuartmorgan-g I will add the StateError.

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