[3.x] Preserve URL fragment across redirects with server-side preserveFragment option

[pascalbaljet]

This PR adds support for the preserveFragment page property on the client side. When the server includes preserveFragment: true in the Inertia page object, the URL fragment from the original request is preserved on the final URL, even across redirects.

Currently, URL fragments are only retained when the request URL and response URL match. This means fragments are lost during redirects (e.g., POST /form redirecting to /result), even though the client knows the intended fragment.

The server-side implementation is in inertiajs/inertia-laravel#829, where you may chain preserveFragment() on a redirect response:

return redirect('/users')->preserveFragment();