`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.
