fix(servo-shell): guard create_app_webview against duplicate session_id

A broadcast LaunchAck can arrive in addition to any direct response,
causing create_app_webview to be called twice for the same session.
Without the guard the second call silently overwrites the existing
WebView entry without destroying it. Early-return if the session is
already tracked.

crates/weft-servo-shell/src/embedder.rs

@@ -133,6 +133,9 @@ impl App {
     }
 
     fn create_app_webview(&mut self, session_id: u64, app_id: &str) {
+        if self.app_webviews.contains_key(&session_id) {
+            return;
+        }
         let (Some(servo), Some(rc)) = (&self.servo, &self.rendering_context) else {
             return;
         };