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/constellation/constellation.rs

@@ -4467,11 +4467,13 @@ where
             },
             // TODO: This should use the ScriptThreadMessage::EvaluateJavaScript command
             WebDriverCommandMsg::ScriptCommand(browsing_context_id, cmd) => {
-                let pipeline_id = self
-                    .browsing_contexts
-                    .get(&browsing_context_id)
-                    .expect("ScriptCommand: Browsing context must exist at this point")
-                    .pipeline_id;
+                let pipeline_id = if let Some(browsing_context) =
+                    self.browsing_contexts.get(&browsing_context_id)
+                {
+                    browsing_context.pipeline_id
+                } else {
+                    return warn!("{}: Browsing context is not ready", browsing_context_id);
+                };
 
                 match &cmd {
                     WebDriverScriptCommand::AddLoadStatusSender(_, sender) => {