[2.x] PHPStan fails in extensions depending on flarum/tags: Flarum\Gdpr\Events\Erased not found #5194

Current Behavior

Since flarum/tags 2.0.1, PHPStan fails in any extension that depends on flarum/tags but doesn't have flarum/gdpr installed:

 -1     Class Flarum\Gdpr\Events\Erased not found.
        🪪  class.notFound
 [ERROR] Found 1 error

#5158 added this to extensions/tags/extend.php:

->whenExtensionEnabled('flarum-gdpr', fn () => [
    (new Extend\Event())
        ->listen(\Flarum\Gdpr\Events\Erased::class, Listener\RefreshLastPostedDiscussionsAfterErasure::class),
]),

It's fine at runtime, since the closure only runs when GDPR is enabled. But flarum/phpstan analyses the dependency's extend.php, and the ::class reference fails when flarum/gdpr isn't installed. extensions/messages/extend.php uses the same pattern, so extensions depending on flarum/messages are affected too.

Steps to Reproduce

1. Take an extension that requires flarum/tags and runs composer analyse:phpstan (e.g. fof/categories).
2. composer update flarum/tags to get v2.0.1.
3. Run PHPStan.

Expected Behavior

Optional integrations in a bundled extension's extend.php shouldn't break static analysis for extensions that depend on it.

Possible Solution

• Reference the event by string in tags and messages ('Flarum\Gdpr\Events\Erased'). Extend\Event::listen() accepts a class name string, and PHPStan won't try to resolve it.
• Or handle classes referenced inside whenExtensionEnabled() closures in flarum/phpstan.

Workaround for now: add flarum/gdpr to the extension's require-dev.

Additional Context

Found on FriendsOfFlarum/categories#10.