fix: fixes offset calculation for paginated Doctrine collections

The code was erroneously using the calculated `$pageCount`, which
represented the total number of pages in the collection, instead of the
`$perPage` value when calculating the result offset. This was due to
missing assertions in the unit tests. The assertions have been added,
and the code corrected accordingly.

src/ResourceGenerator/ExtractCollectionTrait.php

@@ -118,8 +118,8 @@ private function extractDoctrinePaginator(
         return $this->createPaginatedCollectionResource(
             $pageCount,
             $data,
-            function (int $page) use ($query, $pageCount) {
-                $query->setFirstResult($pageCount * ($page - 1));
+            function (int $page) use ($query, $perPage) {
+                $query->setFirstResult($perPage * ($page - 1));
             },
             $collection,
             $metadata,

test/ResourceGenerator/DoctrinePaginatorTest.php

@@ -153,6 +153,9 @@ public function testCreatesLinksForQueryBasedPagination()
             ->method('getMaxResults')
             ->with()
             ->willReturn(15);
+        $query->expects($this->once())
+            ->method('setFirstResult')
+            ->with(30);
         $this->paginator->getQuery()->willReturn($query);
         $this->paginator->count()->willReturn(100);
 
@@ -218,6 +221,9 @@ public function testCreatesLinksForRouteBasedPagination()
             ->method('getMaxResults')
             ->with()
             ->willReturn(15);
+        $query->expects($this->once())
+            ->method('setFirstResult')
+            ->with(30);
         $this->paginator->getQuery()->willReturn($query);
         $this->paginator->count()->willReturn(100);