uSync: merge v17 fixes into v18, v18.1.2 release

Repo: KevinJump/uSync

Merged the two outstanding v17/main commits into v18/main: surfaced the real TemplateOperationStatus on template creation failure (with new serializer tests), and added a non-breaking async callback surface to uSyncCallbacks (#1050)
Released v18.1.2 to NuGet