Add generic return type to AbstractCrudController::getContext()

[ERuban]

EasyAdmin 5 made the CRUD stack generic: AbstractCrudController declares @template TEntity of object, all action methods are annotated with @param AdminContext<TEntity> $context, and the chain continues through AdminContextInterface<TEntity>::getEntity(): EntityDto<TEntity> down to EntityDto::getInstance(): TEntity|null.

One link is missing: getContext() has no @return annotation, so the generic type is lost right at the start of the chain:

/** @extends AbstractCrudController<Product> */
final class ProductCrudController extends AbstractCrudController
{
    public function someAction(): void
    {
        $product = $this->getContext()?->getEntity()->getInstance();
        // PHPStan infers object|null, expected Product|null
    }
}

This forces consumers to keep manual /** @var Product|null */ annotations, although the point of the 5.0 generics (see the "Static Analysis" section in UPGRADE.md) is to remove them.

This PR adds the missing @return AdminContext|null annotation, matching the existing @param AdminContext annotations of the action methods. With it, PHPStan infers Product|null in the example above.

Docblock-only change, no runtime impact.