feat(windows): add DevTools, accelerator key and download settings

[Spoki4]

Problem

WebView2 turns on DevTools, browser accelerator keys and downloads by default, and WindowsWebViewController has no way to turn any of them off:

• F12 and Ctrl+Shift+I always open DevTools, whatever the app wants.
• F5, Ctrl+P, Ctrl+F and the other browser shortcuts always go to WebView2, not to the app.
• A page can always save files to the user's Downloads folder. DownloadStarting is marked handled and the download goes ahead, with no way to refuse it from Dart.

That is fine for a general-purpose browser view, but it blocks apps that show content they do not fully control: kiosks, embedded help, previews of user-supplied HTML. On iOS/macOS the same need is already covered by WebKitWebViewController.setInspectable.

Fix

Three methods on WindowsWebViewController, each backed by a WebView2 setting:

API	WebView2
setInspectable(bool)	ICoreWebView2Settings::put_AreDevToolsEnabled — named after WebKitWebViewController.setInspectable
setBrowserAcceleratorKeysEnabled(bool)	ICoreWebView2Settings3::put_AreBrowserAcceleratorKeysEnabled
setDownloadsEnabled(bool)	DownloadStarting → put_Cancel(TRUE) before a download operation exists, so nothing is written and no download event is sent

final windows = controller.platform as WindowsWebViewController;
await windows.setInspectable(false);
await windows.setBrowserAcceleratorKeysEnabled(false);
await windows.setDownloadsEnabled(false);

Defaults are unchanged. Each method goes through the existing Pigeon host API (setZoomControlEnabled served as the template), and the channels are normalized with tool/normalize_pigeon_namespaces.py.

The controller keeps the requested values and applies them again when a failed initialization is retried (the widget's Refresh), because the new WebView2 starts from the WebView2 defaults. A small private _WebView2Setting also records what the current native WebView already has, so a value is sent once even when the setter and the initialization race.

setBrowserAcceleratorKeysEnabled needs ICoreWebView2Settings3. On a runtime without it the call throws a PlatformException (not_supported) rather than failing silently.

Tests

• webview_all_windows: unit tests for all three settings forwarding to the host API, and for their re-application after a failed-then-refreshed initialization.
• examples/platform: an integration test with a real WebView2. It starts one download with downloads allowed, as a control, then the same download with downloads disabled, and checks the user's Downloads folder. Without the control, a page that never started a download would pass too. The test removes both files.
• The integration test catches the bug. With SetDownloadsEnabled patched to ignore its argument, it fails with "A download started while downloads were disabled."

Verification

Windows 11 Pro 25H2 (26200), WebView2 Runtime 153.0, Flutter 3.47.0, VS 2022 Build Tools 17.14:

• flutter build windows --debug in examples/platform builds with no warnings.
• flutter test integration_test/webview_flutter_test.dart -d windows: All tests passed! (29 passed, 3 skipped), new test included.
• flutter test in webview_all_windows: 53 tests pass.
• flutter analyze --fatal-infos in webview_all_windows and examples/platform: No issues found!
• dart format --set-exit-if-changed on the changed files is clean, and tool/normalize_pigeon_namespaces.py --check all passes.

Docs: platforms/windows.md (EN/ZH) controller API table and the Windows section of guides/debugging.md. I did not touch CHANGELOG.md, since release notes come with the release.

[moluopro]

Thanks!