Promote defaults._title to top level in route attributes

First pass at this, added the new attribute subclass and promoted the title property in system.module routes.

This MR was assisted by Claude Code.

Not sure. _title_callback has accompanying properties that aren't as widely used - _title_arguments and _title_context - so we would likely need to do those as well, and then that feels like a slippery slope. This was intended just as DX convenience for the 90% use case where you are routing a controller or form that has a static title.

There might be some more common ones that we should promote? Maybe worth doing an analysis of *.routing.yml?

In core there are 414 defaults._title definitions. The only thing there is more of is requirements._access, which has 509. There are also 179 requirements._permission. After that there's a long tail, none more than 100. I think if we convert any more it should only be these three at most.

Alternatively, what if we used a separate attribute here, instead of extending Symfony? We could even add other convenience attributes:

#[Route(
  path: '/admin/config/system/site-information',
  name: 'system.site_information_settings',
)]
#[PageTitle(new TranslatableMarkup('Basic site settings'))]
#[Permission('administer site configuration')]

Not sure if this is really better DX or not though.

FWIW Symfony does handle access control with a separate attribute, because it's in a separate package: https://symfony.com/doc/current/security.html

class PostController extends AbstractController
{
    #[Route('/posts/{id}/edit', name: 'post_edit')]
    // 'post' refers to the $post parameter of the controller method
    #[IsGranted('edit', 'post')]
    public function edit(Post $post): Response
    {
        // ...
    }
}

#[PageTitle] could be extended to handle static and dynamic titles from a single attribute:

#[PageTitle(new TranslatableMarkup('Basic site settings'))]
#[PageTitle(callback: 'self::title')]

Also I was wrong in #7 - title_arguments and title_context are actually for adding args/context to title when it is automatically translated, and we sidestepped that by using new TranslatableMarkup() directly (although you can't pass dynamic args now...)

And because this is no longer stored with the routing data we might even be able to use closures to inline dynamic title code:

#[PageTitle(static fn(NodeInterface $node) => new TranslatableMarkup('Edit @label', ['@label' => $node->label()]))]

Having said that for dynamic titles you can also just return #title from the controller...

Updated IS with the two options - a single extended #[Route] attribute or a separate #[PageTitle] attribute.

Added support for title closures to the #[Route] attribute and tested it by converting block_content.routing.yml with a _title_callback to use an inline closure instead.

This is awkward when there is more than one Route attribute on a method, because I'm not sure we know which attribute to look up again, but maybe that's an edge case we don't need to handle.

I guess this is the question here: is the page title tied to the page (so there should only be one), or the route?

We can support multiple attributes - instead of [class, method] we can store [class, method, index] so we know which attribute has the closure. Added this, and some test coverage.

This now has the same capabilities as the existing YAML route declarations, just with additional syntactic sugar for both static and dynamic titles.

If people are happy with the current direction I'll update the IS and write a CR.

Yeah, so that rules out #[PageTitle] being a separate attribute.

Thanks for the review. I accepted the suggestions and wrote a CR: https://www.drupal.org/node/3614321

As per the above issue, closures in attribute arguments are only allowed in PHP 8.5 and above: https://wiki.php.net/rfc/closures_in_const_expr

We need to revert this from 11.x, not sure whether to just keep it in 12 or disallow closures in 11 or something else.

We do have to revert the block_content conversion from 11.x. This also blocks converting the remaining routing.yml in a similar way, but I guess we can just continue that in main only.

We could even keep the test cases and skip on PHP 8.4 and earlier I guess.

11.x backport in MR!16652, keeps the closure functionality but limits the tests to run on PHP 8.5+ only. block_content.routing.yml is still converted but uses _title_callback as before.

Updated the CR to mention closures are only available in PHP 8.5.