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>

components/script/webdriver_handlers.rs

@@ -2086,3 +2086,16 @@ pub(crate) fn handle_remove_load_status_sender(
         window.set_webdriver_load_status_sender(None);
     }
 }
+
+pub(crate) fn handle_get_window_handle(
+    pipeline_id: PipelineId,
+    reply: IpcSender<Result<String, ErrorStatus>>,
+) {
+    if let Some(res) = ScriptThread::find_document(pipeline_id)
+        .map(|document| document.upcast::<Node>().unique_id(pipeline_id))
+    {
+        reply.send(Ok(res)).ok();
+    } else {
+        reply.send(Err(ErrorStatus::NoSuchWindow)).ok();
+    }
+}