fix(importer): keep first occurrence when an artintro page image repeats (#1433)

fix(importer): keep first occurrence when an artintro page image repeats (#1433)

fix(importer): keep first (min n) occurrence when an artintro page image repeats

mwnf3-exhibition-item-importer.ts's importArtintroPageImages wrote one
collection_item pivot row per legacy image row via writeCollectionItem,
which upserts on (collection_id, item_id) via INSERT...ON DUPLICATE KEY
UPDATE. When the same item appears more than once on a page (distinct
`n` grid slots -- e.g. several photos of the same monument), the query
is already ordered by `n` ascending, so whichever occurrence is processed
last silently overwrites the earlier ones -- but legacy always treats the
*first* occurrence (n=1) as the default/featured item shown on page load.

Confirmed concretely in two cases before this fix: the Umayyad Mosque item
on theme 1/page 1 (legacy default) exported with display_order=9 instead
of 1; item M;ISL;es;1;29 on page 9 exported collapsed to display_order=3
instead of 1.

Track seen (collection_id, item_id) pairs and skip subsequent occurrences
instead of re-writing over the first one. Added a regression test
reproducing the Umayyad-Mosque-shaped case (item at n=1, n=5, n=9 on one
page) asserting writeCollectionItem is called exactly once, with the
first occurrence'\''s display_order and picture.

Epic 8 of the islamicart legacy-parity backlog.

Co-authored-by: Pascal HAVELANGE <havelangep@hotmail.com>