Catch and discard JSDisconnectedException in IJSObjectReference.DisposeAsync for WebView

[Copilot]

When components dispose IJSObjectReference instances during DisposeAsync, a JSDisconnectedException can be thrown if the JS runtime is no longer available (circuit disconnected, WebView navigated away, or WebView disposed). This is a spurious error — the JS side is already gone, so server-side cleanup of the reference is unnecessary.

Description

JSObjectReference.DisposeAsync already catches and discards JSDisconnectedException. This PR extends the same pattern to Blazor WebView:

• WebViewJSRuntime: Added _disconnected flag + internal MarkAsDisconnected() method. When set, BeginInvokeJS throws JSDisconnectedException immediately (same pattern as RemoteJSRuntime.MarkPermanentlyDisconnected()).

• PageContext.DisposeAsync: Calls JSRuntime.MarkAsDisconnected() before Renderer.DisposeAsync(), preventing JS interop calls from hanging or throwing during component teardown when the page has navigated away or the WebView is being disposed.

• Tests: Two new tests verify that the JS runtime is marked disconnected on WebView disposal and on page navigation.

// PageContext.DisposeAsync — before this change, DisposeAsync on a JSObjectReference
// would hang or propagate exceptions when the WebView was already gone
public async ValueTask DisposeAsync()
{
    JSRuntime.MarkAsDisconnected(); // new
    await Renderer.DisposeAsync();
    await _serviceScope.DisposeAsync();
}