ProxyBuilder generates invalid \static return type causing PHP Fatal error

Problem/Motivation

The `ProxyBuilder` class generates invalid PHP code when a method has `static` as return type (PHP 8+ feature). It incorrectly outputs `\static` instead of `static`, causing a PHP Fatal error:

PHP Fatal error: '\static' is an invalid class name in .../ProxyClass/Hook/ContentHooks.php on line 73

In `core/lib/Drupal/Component/ProxyBuilder/ProxyBuilder.php`, lines 239-242, the code checks if the return type is not built-in and prepends a backslash:

php
if (!$return_type->isBuiltin()) {
  // The parameter is a class or interface.
  $signature_line .= '\\';
}

The `static` return type is not a built-in type (`isBuiltin()` returns `false`), but it's also not a class/interface - it's a PHP keyword that should not have a backslash prefix. The code already handles `self` specially (line 244-246), but `static` was not considered.

Steps to reproduce

1. Have a service class that implements `ContainerInjectionInterface` with the `create()` method using `static` return type:

php
class ContentHooks implements ContainerInjectionInterface {
  public static function create(ContainerInterface $container): static {
    return new static();
  }
}

2. Mark this service as lazy in `*.services.yml`

3. Run the proxy class generator:
bash
php core/scripts/generate-proxy-class.php 'Drupal\eca_content\Hook\ContentHooks' "modules/contrib/eca/modules/content/src"

4. Try to clear cache with `drush cr` - PHP Fatal error occurs

**Environment:**
- Drupal 11.3.2
- PHP 8.4.16
- Debian GNU/Linux 13 (trixie)
- Apache/2.4.66

**Affected module:** ECA 3.0.10 (eca_content submodule)

Proposed resolution

Add a check for `static` return type similar to how `self` is handled:

diff
if ($reflection_method->hasReturnType()) {
$signature_line .= ': ';
$return_type = $reflection_method->getReturnType();
+ $return_type_name = $return_type->getName();
if ($return_type->allowsNull()) {
$signature_line .= '?';
}
- if (!$return_type->isBuiltin()) {
- // The parameter is a class or interface.
+ if (!$return_type->isBuiltin() && $return_type_name !== 'static') {
+ // The return type is a class or interface.
+ // Note: 'static' is not built-in but also not a class, it's a PHP keyword.
$signature_line .= '\\';
}
- $return_type_name = $return_type->getName();
if ($return_type_name === 'self') {

Remaining tasks

- Review and commit patch
- Add test case for `static` return type in `ProxyBuilderTest.php`
- Regenerate affected core proxy classes (if any)

User interface changes

None.

Introduced terminology

None.

API changes

None. This is a bug fix - the API remains the same, but now correctly handles `static` return types.

Data model changes

None.

Release notes snippet

Fixed a bug where the ProxyBuilder would generate invalid PHP code (`\static` instead of `static`) for methods with `static` return type, causing a PHP Fatal error when using lazy services with `ContainerInjectionInterface::create()` on PHP 8+.