webdriver: Implement the "Get Window Handles" command (#38622)

Implment get window handles according to
[spec](https://w3c.github.io/webdriver/#dfn-window-handles).
- Window handles are supposed to identify `browsing context`. However,
based on `get window handle command` and `get window handles command`,
we only need to care about top level browsing context.
- Back then, we use a random generated uuid for eacch webview id, it is
not correct but still work because all commands depend on `webview id`
and `browsing context id`. The only case we need window handle is is
when webdriver gets window object with js script. Since the object is
converted to the id of window's document node, `get window handle`
should return the same thing.

Action run (with updated expectation):
https://github.com/longvatrong111/servo/actions/runs/16957610535
https://github.com/longvatrong111/servo/actions/runs/16957612027

Some tests may sporadically timeout due to unstable hit test.

cc: @xiaochengh

---------

Signed-off-by: batu_hoang <hoang.binh.trong@huawei.com>

ports/servoshell/desktop/app.rs

@@ -381,6 +381,17 @@ impl App {
                         running_state.set_pending_focus(focus_id, response_sender);
                     }
                 },
+                WebDriverCommandMsg::GetAllWebViews(response_sender) => {
+                    let webviews = running_state
+                        .webviews()
+                        .iter()
+                        .map(|(id, _)| *id)
+                        .collect::<Vec<_>>();
+
+                    if let Err(error) = response_sender.send(Ok(webviews)) {
+                        warn!("Failed to send response of GetAllWebViews: {error}");
+                    }
+                },
                 WebDriverCommandMsg::GetWindowRect(_webview_id, response_sender) => {
                     let window = self
                         .windows