fix: volume gap filling for chapter transitions between different volumes

`fillVolumeGaps` incorrectly left chapters null when surrounded by two
different non-null volumes (e.g. Vol10 → null → Vol11). Simplify the
condition to always prefer the previous volume, covering all cases.

Also fix `InMemoryMangaRepository::findExistingChaptersByNumbers` to
return an array keyed by chapter number, matching the Doctrine contract.

Add 5 unit tests for MangadxChapterSynchronizationService covering
volume transitions, start-of-series gaps, explicit volumes, FR/EN
priority, and deduplication of existing chapters.

tests/Domain/Manga/Adapter/InMemoryMangaRepository.php

@@ -267,10 +267,14 @@ class InMemoryMangaRepository implements MangaRepositoryInterface
             return [];
         }
 
-        return array_filter(
-            $this->chapters[$mangaId],
-            fn (Chapter $chapter) => in_array($chapter->getNumber(), $chapterNumbers)
-        );
+        $result = [];
+        foreach ($this->chapters[$mangaId] as $chapter) {
+            if (in_array($chapter->getNumber(), $chapterNumbers)) {
+                $result[$chapter->getNumber()] = $chapter;
+            }
+        }
+
+        return $result;
     }
 
     public function findByMonitoringCriteria(MonitoringCriteria $criteria): array