fix(downloads): SABnzbd category filter hid tracked jobs reassigned to Default, stranding them unimported

[t3chnaztea]

The bug

SABnzbd silently reassigns jobs with unknown categories to Default (*). Listenarr sends cat=<configured> on add, but every read mapper filtered slots by configured category before matching nzo_id (SabnzbdResponseMapper.MapQueueSlotToQueueItem / MapHistorySlotToQueueItem). So when the configured category doesn't exist in SABnzbd, a job Listenarr itself grabbed becomes invisible to it: the monitor reconciles by nzo_id but never sees the slot, the download shows "1 tracked but 0 in client", and it never progresses or imports. Notably the history endpoint is already fetched unfiltered — the recovery data was in hand and discarded client-side.

The fix

Two parts, mirroring how Radarr treats categories (reconcile items you grabbed by download ID; category scopes discovery, not tracked items):

1. Tracked nzo_ids bypass the category filter. The fetch workflow already builds the monitored-id set; it now flows into both QueueItem mappers as an optional parameter, and a slot whose nzo_id is tracked is returned regardless of its category. Untracked discovery/display filtering is unchanged. (MapQueueSlotToDownloadClientItem is deliberately untouched: the item-fetch path has no monitored set and isn't the strand path.)
2. Connection test warns about missing categories. After the version check passes, the tester probes mode=get_cats and, if the configured category is absent, returns an advisory ("category 'x' does not exist in SABnzbd. Jobs will fall into Default and may not import. Create the category…") — still a passing test, and the probe is best-effort (any get_cats failure degrades to a clean pass).

Tests

• QueueSlot_InDifferentCategory_ButTracked_IsReturned / HistorySlot_InDifferentCategory_ButTracked_IsReturned
• QueueSlot_InDifferentCategory_NotTracked_IsFiltered / HistorySlot_InDifferentCategory_NotTracked_IsFiltered (discovery filtering preserved)
• TestConnectionAsync_CategoryMissingFromSabnzbd_ReturnsAdvisoryPass / TestConnectionAsync_CategoryPresentInSabnzbd_ReturnsCleanPass
• One existing test updated: Sabnzbd_GetQueueAndItems_FilterByConfiguredCategory passed both nzo_ids as tracked yet asserted the foreign-category one was hidden — it encoded exactly this bug. Its intent (respect configured category) is a discovery concern, so it now exercises the no-tracked-ids path and keeps its assertions.

Full suite green: 1195 passed / 0 failed; dotnet format --verify-no-changes clean.

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