Close session before freeing notification

With the new CookieStore, the session must be freed before the notification is.
This is how it works in CDP, but in fetch, we were pretty lazy about it. This
caused the notification to be freed first, and then the cookiestore to try to
unregister: UAF.

src/lightpanda.zig

@@ -67,6 +67,7 @@ pub fn fetch(app: *App, browser: *Browser, url: [:0]const u8, opts: FetchOpts) !
     defer notification.deinit();
 
     var session = try browser.newSession(notification);
+    defer browser.closeSession();
 
     if (app.config.cookieFile()) |cookie_path| {
         cookies.loadFromFile(session, cookie_path);