Only export content/element items once per save-and-publish

[KevinJump]

What

uSync exported a content or Library (Element) item twice for a single save-and-publish, and three times for a save that un-published a culture. This fixes it so each item is exported once per Umbraco operation.

Why

Umbraco 18.1 raises ContentSavedNotification for a save-and-publish as well as ContentPublishedNotification (umbraco/Umbraco-CMS#23523, PR #23528).

Worth noting for review: the upstream PR was raised against release/17.6 and patches ContentService.CommitDocumentChangesInternal, so the diff makes it look Document-only. In v18 that method lives in PublishableContentServiceBase, shared by Documents and Elements — so on release/18.1 the extra notification fires for both, and ElementHandler had the same problem as ContentHandler.

ContentHandler and ElementHandler each listen for the saved and the published/unpublished notifications. The <remarks> on those handlers explained why — "some publication events do not fire the save notification" — and that premise is what broke.

How

Every notification raised by one Umbraco operation carries the same notification state instance — PublishableContentServiceBase raises them all with .WithState(notificationState), sourced from savingNotification.State. So ProcessItem now claims each item in that shared state (a HashSet<Guid> under a new uSync.EventExportedItemsKey) and skips it if an earlier notification in the same operation already exported it.

Three decisions worth a look:

• First notification wins, i.e. Saved exports and Published skips. Safe because SaveContent(content) runs before the Saved notification is raised, and the publish values were applied before that — the item is already in its final published state, on the same instance both notifications carry.
• Claim before export, not after. A failed export isn't retried by the next notification in the same operation; it would almost certainly fail again and add a second warning to notification.Messages.
• Lock around the get-or-create. Dispatch within a scope is sequential so this is belt-and-braces, but the state dictionary is a plain Dictionary and SyncScopedNotificationPublisher can dispatch on a queued background thread when BackgroundNotifications is on.

SavedNotification handling for these two handlers is now routed through ProcessItem via an override, rather than the inline copy of the export loop in SyncHandlerRoot — that copy bypassed the de-duplication. It's untouched for every other handler.

MediaHandler derives from ContentHandlerBase, not PublishableContentHandlerBase, and has no publish notifications, so it's unaffected.

Behaviour

Action	Before	After
Save-and-publish	2 exports	1
Save-and-publish un-publishing a culture	3 exports	1
Publish / un-publish from tree	1	1 (unchanged)
Plain save	1	1 (unchanged)
Publish with descendants	1 per doc	unchanged
uSync import	0 (events paused)	unchanged

Testing

New uSync.Tests/SyncHandlers/ExportDeduplicationTests.cs covers the save-and-publish pair, the save+unpublish pair, publish-with-no-save, per-item claiming across a branch publish, re-claiming in a later operation, and that uSync's own notification state keys survive.

Full solution builds clean; all 190 tests pass (6 new).

Not included

Both came out of the same review of the Umbraco 18.1 changes, but are behaviour changes rather than bug fixes, so they're deliberately left out:

• ContentSavedNotification.SavedCultures / ContentPublishedNotification.PublishedCultures (#23313) would let uSync export only the affected cultures, and skip no-op saves.
• PublishableContentBaseSerializer still does save-then-publish as two operations rather than the new single-scope IPublishableContentService.SaveAndPublish (#23277).

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