webdriver: Focus browsing context when switch frame (#39086)

#38889 adds back the mechanism to focus the window when [switch to
window](https://w3c.github.io/webdriver/#switch-to-window). After that,
it causes many flaky TIMEOUT. Turns out the real reason is same as the
phenomenon which I thought was unrelated:
https://github.com/servo/servo/pull/38889#issuecomment-3217512339: we
need to focus the browsing context as well according to spec. This is
important for Servo because it relies on
f4dd2960b8/components/constellation/constellation_webview.rs (L64)
to determine which pipeline to send `InputEvent` to.

Testing: Before, there are 9 ~ 13 flaky results. [Before
1](https://github.com/yezhizhen/servo/actions/runs/17379170889), [Before
2](https://github.com/yezhizhen/servo/actions/runs/17392219417), [Before
3](https://github.com/yezhizhen/servo/actions/runs/17379172612).
Now there are 2 ~ 4 flaky results. [After
1](https://github.com/yezhizhen/servo/actions/runs/17394359570), [After
2](https://github.com/yezhizhen/servo/actions/runs/17394358218), [After
3](https://github.com/yezhizhen/servo/actions/runs/17394357400).
Fixes: https://github.com/servo/servo/pull/38889#issuecomment-3218600566
Fixes: #38906, which is last blocking point to enable WebDriver CI.

---------

Signed-off-by: Euclid Ye <euclid.ye@huawei.com>

components/constellation/constellation.rs

@@ -4524,6 +4524,9 @@ where
                 let is_open = self.browsing_contexts.contains_key(&browsing_context_id);
                 let _ = response_sender.send(is_open);
             },
+            WebDriverCommandMsg::FocusBrowsingContext(browsing_context_id) => {
+                self.handle_focus_remote_document_msg(browsing_context_id);
+            },
             // TODO: This should use the ScriptThreadMessage::EvaluateJavaScript command
             WebDriverCommandMsg::ScriptCommand(browsing_context_id, cmd) => {
                 let pipeline_id = if let Some(browsing_context) =