Fix attachment visibility check

[garethrees]

Update visibility checks for downloading zip files of batch requests now that an FoiAttachment can have prominence set independently of the parent IncomingMessage.

[gbp]

Not tested - but I can't see any issue with this change.

[garethrees]

# BEFORE

$ tree batch-1-exdfbginuyvt-export-2026-03-24-143017
batch-1-exdfbginuyvt-export-2026-03-24-143017
├── .DS_Store
├── batch-1-exdfbginuyvt-dashboard-2026-03-24-143017.csv
├── Department for Humpadinking - exdfbginuyvt
│   └── 2026-03-24-135805
│       └── outgoing_12.txt
└── Geraldine Quango - exdfbginuyvt_2
    ├── .DS_Store
    ├── 2012-01-08-091615                   <<---------------------- HIDDEN ATTACHMENT PRESENT
    │   ├── .DS_Store
    │   ├── attachments-8
    │   │   └── fs 50379341.pdf 
    │   └── incoming_8.txt
    └── 2026-03-24-135905
        └── outgoing_13.txt

7 directories, 8 files

# AFTER

$ tree batch-1-exdfbginuyvt-export-2026-03-24-163225
batch-1-exdfbginuyvt-export-2026-03-24-163225
├── .DS_Store
├── batch-1-exdfbginuyvt-dashboard-2026-03-24-163225.csv
├── Department for Humpadinking - exdfbginuyvt
│   ├── .DS_Store
│   └── 2026-03-24-135805
│       └── outgoing_12.txt
└── Geraldine Quango - exdfbginuyvt_2
    ├── .DS_Store
    ├── 2012-01-08-091615                   <<---------------------- HIDDEN ATTACHMENT NOT PRESENT
    │   └── incoming_8.txt
    └── 2026-03-24-135905
        └── outgoing_13.txt

6 directories, 7 files