Fix TestCampaign sending only subscribers, failing listmonk validation

listmonk's /api/campaigns/:id/test handler binds the request into the
same campReq struct create/update use and validates it in full, so a
request with only {"subscribers": [...]} fails with "Invalid length
for name" before subscribers is ever read. TestCampaign now sends the
campaign's full CampaignInput alongside subscribers.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

internal/campaign/sync.go

@@ -137,7 +137,7 @@ func syncOne(lm *listmonk.Client, slug, dir, path string, defaultPreviewEmails [
 	}
 	var previewErr error
 	if len(recipients) > 0 {
-		previewErr = lm.TestCampaign(campaignID, recipients)
+		previewErr = lm.TestCampaign(campaignID, input, recipients)
 	}
 	return syncOneResult{Changed: true, PreviewErr: previewErr}, nil
 }