Fix relevance sort (#2773)

- Adds a field to QueryCriteria that determines whether the sort provided is the controller's default sort
- Set this field to true iff sort not in query params. Default it to false
- Override $sort if a new default sort has been set on search state, and the param is true.
- Add tests!

src/Api/Controller/AbstractSerializeController.php

@@ -12,6 +12,7 @@ namespace Flarum\Api\Controller;
 use Flarum\Api\JsonApiResponse;
 use Illuminate\Contracts\Container\Container;
 use Illuminate\Database\Eloquent\Collection;
+use Illuminate\Support\Arr;
 use Illuminate\Support\Str;
 use Psr\Http\Message\ResponseInterface;
 use Psr\Http\Message\ServerRequestInterface;
@@ -255,6 +256,11 @@ abstract class AbstractSerializeController implements RequestHandlerInterface
         return new Parameters($request->getQueryParams());
     }
 
+    protected function sortIsDefault(ServerRequestInterface $request): bool
+    {
+        return ! Arr::get($request->getQueryParams(), 'sort');
+    }
+
     /**
      * Set the serializer that will serialize data for the endpoint.
      *